This is open to interpretation or preference, as well as where iit is being built. Our is 20 inches off the floor for the bottom loop as we preferred this level to be level with the window sill. The second level is about 7-8 inches above that when you had the plywood and Styrofoam. We believe this makes for great village and train watching. The upper level has one o-gauge train loop,and the bottom level has one standard gauge and 1-2 o gauge loops depending on how I run the track. Pics are in the 2015 Christmas layouts on the forum here or on Youtube by searching Joe Gozzo
Hope this helps,
Joe