I just saw this post for the first time and given that you want to use a GSHP, the best thing you can do with tubing is to keep it tight because that means a lower water temp and therefore a higher HP efficiency (or if you want to use solar, a greater solar resource).

if loads are low and floor coverings non existent or not resistive (tile) tighter on centers aren't much help. tubing is cheap though. don't skimp if you're guessing.

There are some good heat loss analysis, passive solar design (including roof overhang design), hydronic radiant floor heating design, and integrated heating system performance calculators at this website:

http://www.borstengineeringconstruction.com/Calculators.html

There are also calculators for water work projects (e.g., cross flow turbine, water wheels, hydraulic ram pumps, ditch/pipe flow rate capacity, etc.)