I put plastic sheet down, then 2 inch foam, rebar on chairs, tie pex tube to rebar.

I do all 300 foot runs, keep hot half of run by the outer side of the building and use the last 150 feet in the center of building then back to the manifolds. 

300 is what I was told and have used on 3 shops.  But if the building size, 250 or 350 would work better, I would use that.  I would keep them all the same lenght though.