You really don't want your cool side to drop much less than 77 degrees, as you want to keep your ambient temps in the low 80's if at all possible.
Not sure if you mentioned it or not, but a UTH should be controlled by a thermostat or dimmer, because they could get too warm.
Samson "might" park himself in the warm side if he's too cool, but if your hides are not identical on each side, he may choose a favorite hide (security) over thermo-regulating.
I do keep my water dish in the middle of the warm and cool sides.
Another alternative is to use a plastic tub, rather than a glass enclosure - they tend to hold heat and humidity easier than glass does.