1. You gave square footage of the floor, the ceiling should be the same. you also need square footage of the walls and what the R values in walls, ceiling and under floor? If this basement is entirely below ground, earth temp is probably in the mid 50's, that is why it used to be 55F down there before insulation
If there are any exposed walls(daylight basement) you need outside design temp to figure heatloss thru that insulated barrier. With this info you can estimate how much heat/energy you need to dump into the space to counter the losses and keep it at say 72F. I find this easiest by converting R value to U factor. R is the resistance to transfer, and U is the actual transfer rate. Because of this, they are the inverse of each other. To invert a number you divide it into 1. So if you have say R10 under the floor heat system, 1 divided by 10 = a U factor of 0.1.
Once you know U factor, it is simple as U = BTU/hr ,per sq/ft, per degreeF of temp difference. If you want 72F in the room, and the earth under the floor is 55F, that = 17F temp difference. The equation to figure your floor with R10 will be 0.1BTU X 1535 SQ/FT x 17F temp difference or 2610 BTU/HR lost thru the floor at 72F room temp Since heat only flows from hot to cold, you need a temp difference to transfer energy. When heating the room from the floor, you must heat the floor to greater than the room temp to get transfer into the room. This added temp will increase your downward heat loss, but you can recalculate the floor at the higher temp once you get a better idea what temp you need to run to replace the energy lost thru the rest of the surface areas at their total u factors. It will be no less than the basic number calculated this way using your actual R values, so start with that basic number for the floor.
Say the ceiling is R30. That = 0.33U. It is roughly the same 1535 SQ/Ft, maybe minus stairwell? But since it is heated up there, the temp difference is far less. If you use forced air heat upstairs, lets say the floor runs around 68F. 0.33BTU X 1535 SQ/FT X 4F = 204 BTU/HR lost thru the ceiling at a 72F basement temp.
Guestimate for a 1535 SQ/FT, square, 8' tall basement @ R19 wall: .052BTU X 12160 SQ/FT X 17F = 10,879 BTU/HR thru the walls @ 72F room temp... YBMV
Any window areas are handled the same way, except using the outside design temp...
Repeat this process for each different surface and temp difference then total the BTUs. That is the basic heat load of the basement. This will give you an idea how much energy you need to transfer from the floor into the room to keep the room at your desired temp. Also keep in mind every material in the makup of the envelope(drywall, insulation or studs, sealant, basement masonary, ect) between the heated space and "outside" the heated space has an R value. You need to total all these values before converting to a U factor. Also a wall makup will have a bearing on heat transfer. For instance if you put up a 2X4 stud wall over the masonary basement walls to hold your new insulation and finish out the space, the studs have a different R value and will conduct energy differently than the cavity full of insulation right beside it. A 2X4 wall 24"OC has roughly 7% structure material and 93% insulated cavities. You can add up 2 total R values, one for studs and one for insulated area and divide the wall area up 93% to 7% and apply these total R values to the appropriate wall area. Breaking the wall area up will make your heat loss estimates a little more precise, but might not be necessary to get you to some useable information.
So, do your math homework and post back here a total BTU number, then we can discuss the next step.
Good Luck if you should decide to accept this mission