Playing devil's advocate here : I'm using TeptileBasic's heat tape : 12" is 23W/ft and 4" is 11W/ft. So,4 shelves will each have 23W and 3 shelves will each have 33W. That makes only a difference of 8W. From an electrical standpoint, are you saying that is significant ?
From a tub "warm spot" viewpoint... each tub on all levels will have about 25% belly "warm spot" coverage. That seems like a good ratio. Thoughts ? and THANKS !