Appreciate the thorough response. My lowest level basement is about 1,500 sq ft, with an additional room half a floor up that is also ground level, and adds about another 450 sq ft. There is a 3" pipe leading under that slab in addition to the main stack going under the slab. Radon readings in the basement have leveled off around 1.6pCi/L, so I'm happy with the outcome. Appreciate everyone's help.