Instead of a dimmer, I'd recommend putting any CHE on a thermostat. That way you're 100% sure that you're in control of the how much heat you're providing.

Personally, I only provide ambient temps of 80-82 with Radiant Heat Panels (RHPs)in my fully enclosed PVC enclosures and have been pretty successful with that.

It's a lot easier to provide stable temps in an enclosed enclosure. With glass tanks you're always going to have fluctuating temperature depending not only on how you're heating it but also the room temps.