The general rule of thumb I see is that the conductance "up" should be 4
times greater than down. I suppose that's a loss to ambient of 20%.
How that would work in your floor is a concern. It seems to me that
the conductance into the floor would not be good, and that into the
ground would be high.
I would think that you'd want to keep the radiant out of the ground.
Some kind of fin tube perhaps. Run it under your beds or tables, if you
wish. The higher the temperature water circulating the less surface area
you will need.
The heater design