That would make sense. However, if I use 6" on my 28Qt in the same fashion would that provide too little of a gradient for the smaller size?
I don't think so. The 28-qt is 23 inches long so 6 inch tape would leave plenty of unheated floor space available if the snake wanted a place to cool off. Remember a UTH really don't do a whole lot for ambient temps, and your room temps are 65-70*F.