I've never thought to temp check the underside of my tubs to be honest. My rack has a recessed grove for the flexwatt so my tub never actually touches the heat tape.

Getting a UTH for the cool side will help a great deal with your temps as long as your room temps are constant and don't vary much; otherwise tweaking with the dimmer will drive you crazy.

Like mentioned above, belly's of bps are not very sensitive to pain so its very likely that a snake will burn its belly even when provided with a cooler option..

Good luck tweaking your set up. If I can manage with ambients of 68 you'll figure something out