Think about it. Back heat doesn't radiate outward towards the tubs. Heat rises and most of the heat leaves the level you want to heat and escapes towards the next level. So, by the time you get to the top of 5 or 6 tubs, the bottom tub is going to be 10 degrees/or more cooler than the top few tubs. I've never seen how back heat can effectively work on any reptile rack.

Just my opinion,
Jim Smith