I solve the ambient temp vs. belly heat with a heat lamp & dimmer switch.

UTHs (in my experience) don't produce enough ambient heat to get that 78-82 F ambient you need on the cool side. So I have my UTH, my heat lamp/dimmer combo, and my dual temp thermometer in the tank. I use the Hydrofarm to dial in the UTH (it's currently set at 100F and the hot side temp is at 92 F). I then use the lamp dimmer to dial in the ambient temperature to 78-82 F. Worked all winter, and even managed to get full-piece sheds with the Heat lamp going.

I haven't needed to worry about the temperatures in almost 3 months, as the thermometer tells me that the hot spot and ambient are both indicated.