First, do you have a water bowl in the rub yet or not? If not put one in for the testing.

Second, is the temperature you're measuring ambient temp or is it the surface temp over the heat pad?

I've found with a hatchling-sized tub that putting a small hide over the heat and the water dish on the cool side works well and I really don't have to spray much at all. The cool side "hide" is either the top of the warm side hide, or it's a tight corner of the tub closest to the water bowl.