I am planning a retrofit to my heating system, which today consists of two wall-mounted
has heaters, installed probably 60+ years ago when the house was build.

The house is on slab on grade. Temperatures are mild, almost never bellow freezing.
We never heat the house at night, and during the day lowest temps are in the high 30's.
First floor has 2,200 sq.ft. and I'd like to have 4 spare circuits for possible attic expansion.

After a few months of web reading I came up with a floor sandwich:
http://www.beffa.com/FloorCrossSection.pdf

and a layout:
http://www.beffa.com/Radiant_Heat.pdf

I still need help with the design.

I would appreciate any critique or advice. I would not mind paying for competent advice, but so far I was not able to cut through the thick, it's just too much info out there.