We are building a super insulated shop(4000 sq ft) with a concrete slab floor with radiant floor heat. thermal calculations and recommendations from American Concrete Institute indicate we should put 3" of extruded polystyrene insulation and a vapor retarder topped by 8" of gravel under the 6" floor. What tolerances are really required on the grade flatness under the insulation? Our sub-contractor is is charging a lot for a very tight tolerance. also, would 3 layers of crossed 1" foam be better and more forgiving than 1 3" layer?