I have a 1200 sq. ft. building with approx 1200 ft of 1/2" pex in the isulated slab. I have an 11 kw boiler and a 50/50 mix of glycol. The manifolds we are using have temperature wells, if we have 120 degree fluid going into the slab what would you expect the fluid to be comming out of the slab before being reintroduced to the boiler?

it depends. I hope you have more than 1 loop, here?

the answer to that question depends on your heat load, how far into your heat demand you are, flow rates, finish coverings, loop lengths, etc.

typical design targets are 10 to 30 degree drops depending on whether this is residential or commercial. but those are "design day" targets. They are typically much lower. Except starting up a demand on a cold slab, it can be much higher until the slab warms up.

I have 4 loops approx. 275' ea. 1/2" pex. Just before the thermostat is satisfied at 68 degrees I have 115 degrees on the supply header and 85 degrees on the return. I should note that my foundation on the outside of the building is still exposed and unisulated, the ground is rock hard and I can't finish the outside work for a few more weeks. The inside is cement right now but will be covered.
I guess my main concern is do I have enough flow? I used some cheap stainless steel headers with flow gauges and they barely move. When I did a calculation for a circulator I figured I had 9' of head and would need 7 to 8 gallons per minute. The pump I used is a B&G nrf22. It seems like two loops are working well and two are lagging.

try purging air from the slow loops or balancing... usually if there are flow meters they double as balancing valves, or there are balancing valves on the other header.

I hooked up my 1/2 hp pump and perged the lines, the first loop did have a little air, the flow meter showed my pump was pumping 1.2 gal/min the second loop had no air and the pump is pushing 1.6 gal/min, the third line has no air in it and it is at 1.4 gal/min, the last line had no air in it and the pump is pushing 1.2 gal/min according to the flow meter. I did each loop individually and ran the pump for 10 minutes per loop. Just for the heck of it I took a gallon jug and a stop watch to see how acurate these flow meters are, the first loop was reading 1.2 gal/min on the flow meter but actually filled a gallon jug in 30 seconds, all the other loops filled the gallon jug in 30 seconds as well. I think I might be better off taking floor temps around the building and try to balance the manifold that way. The floor temps are anywhere from 71 to 66 degrees, the back third of the building has the lowest floor temps. Their's two bedrooms back there and one loop running in that area but also those rooms have more outside footing area that is not insulated, I think the flow meters are junk and things are working well, it just needs a little fine tuning to even things out.

There is no radiant floor heating application in the lower 48 that requires a true 50/50 mix of propylene glycol - short of commercial snow-melting systems.
Your pump is undersized.

This can be avoided with a proper radiant floor design up front.

The floor "temps" are of no value really. I would not worry until the perimeter is insulated.

When choosing the circulator I did a head loss calculation and sized the pump, the B&G nrf-22 performs best at 10 gal/min, I figured I would be about 8 gal/min. Did I calculate wrong? Which pump would you recomend? The glycol mix might be on the strong side but since this is a seasonal cabin that may or may not be used during the winter months I thought a stronger mix would be better, we plan on leaving the thermostat at 39 degrees during the winter months when not in use. I think your right, I should wait until I have the perimiter insulated before futher review of the system.

I have the outside insulated now, and found a super deal on a B&G NFR-36 pump, I should get some serious flow on high speed.

Northern commercial specifications have some 34% glycol for standing hydronic loop considerations because of  a freezing point above a bursting point. Could this help you (?)  BTU's are a product of (= to) a temperature-difference X (a constant I will guess at with 34% glycol by volume of) 470 X GPM gal/minute flowing. Having used NRF36 recently, from a similar slow flow of an undersized grundfos-58, and that was on 7 zones of similar tubing, the temperature difference dropped by nearly 45%. Later it seems better as we added a mixing valve to regulate a portion: a 2-piped slab-section garage floor. That was set at only about 74 for keeping things above 50 at about knee high, in winters that are frequently below zero. The garage doors aree tight - in comparing to some others.

after changing pumps my teperature difference dropped 10 degrees in the mainifolds, I have the nrf-36 set on the medium speed and according to the temp gauges on the headers I have a 18 degree difference. I can turn the pump up to hi speed but how much flow do I need?
I also covered the floor with carpet (.8 R-value) the carpet installer used the wrong pad ( 1.35 R-value) The pad I chose was solid rubber with a .4 R-value but for some reason I didn't get that pad. How much harder will my boiler have to work because of the floor covering? Should I make them replace the pad?

Yeah, make them change the pad. Putting an extra R 1.0 or so on top of an insulated slab pushes a huge amount of heat out the bottom of the slab.

One GPM at a 20F deltaT will give you 10,000btu. If you have 4 loops at 275' you will need 4 gpm@maybe 6' head plus other piping (at heat load). That is not a big pump so I think a UPS 15-58 would be good (or equiv). Going above 250' loop length does increase your head loss a bit.

the carpet changes your delta-T as well... your slab doesn't release heat as quickly with R on top. I am still not clear on what problem you think you are solving with this pump stuff.

If you have a drain located at where the pump and some known pressure drop is at another drain (hose fitting the end) then you can get a hose-cap adapted (some stores already carry this) adapted to a low-range 0-25 or 0-30 psi gauge and read the pressure differences between , -around the pump and check with B&G about
1- at known % antifreeze in solution, 2) What would the difference mean on gauge psig  ie if one was 14 psi and the other was 11.5 a difference of 3.5 pounds (trying to get to a 1/4 pd reading or more accurately).

with water the 1 gpm x (constant ~) 500 x 20 is 10,000 btuh
with 20% glycol that constant drops below 485, and so on, since the heat capacity of the system will be lower per gallon, a calculated heat content flowing by would be lower than just using water at a same temp-differential

if 18 deg differences WORK at a particular WATTS (the volts x amps read at the pump) then try HI NRF speed and read those watts (for fun) vs a new temp-differential and see what heats all if better than before for what time for what change in watts. we might see leaving the bathroom light off appropriately is more important..... 

THEN get ICFH-suggested-other-pad.

since we do not know about what pipe runs may still have some air bubbles in them nor the spacing, and other things, it is a guess that a higher flow will only help and moving from the 58 to the nrf was good enough at medium, but might still for a couple other of reasons get more efficient distribution of the heating off the pipes overall at little power additions to flow and overall less power/energy losses relative to circulation increases. it is a "mean" temperature thing - and a lower r-pad is better as others mentioned.

Too much the clinic stuff -  but maybe check by knowing the ID of the pipe, -and see if that you ever made the flow over a velocity of 2.2 for water and near 3 ft per second if over 40% glycol, as in a flush of each line. Chart or manufacturer will show gpm for flow or pressure drop to achieve for all of that (again that hose-cap-adapter gauge, if needed-, -fast fill short-flush from feed line pressure may be great enough.)

Temperature differential that is high may be required on condensing gas boilers needing to get below 125 at the entering in the boiler so it does condense a lot.

In radiant floor heating systems, floor coverings are critical. Make him change the pad. If you don't, your system will require a higher average design water temperature. Any heat source will require more fuel to maintain the same indoor air temperature and you will pay more to heat your house. In extreme cases, especially where a proper Manual 'J' heat load has not been performed, the house may not even come up to temperature on the coldest days of the year. We design radiant floor systems every day and re-design even more. Many times floor coverings are the problem. Practically any kind of wood or carpet can be used over radiant floors but someone must do the math. As for radiant floor manifold temperatures and temperature differentials, the numbers are affected by floor coverings, pumps, loop lengths and most importantly, the type of radiant panel employed. PEX, copper tube, steel pipe will change heat transfer in a concrete slab and copper tube in plaster ceilings will have a different heat transfer rate for a given area than PEX tubing with aluminum emission plates in a new radiant ceiling under drywall. The Delta T or temperature drop will vary with application and conditions. The magic 20° Delta T, is not. If the thermostat is satisfied on the coldest few days of the year and you are comfortable, the pump and manifold temperature is right. If you don't want to guess, hire a professional to do the math. If you have a radiant floor driven by a condensing boiler the best designs maximize the temperature drop to the boiler, while satisfying occupant comfort at once.