Has anybody run across any figures or formulas relating to the ambient air temperature in a given space and the wattage/area of a UTH pad?

I'm trying to figure out which UTH to go with for an Exo-Terra
18"x18"x18" terrarium with varying thicknesses of a mix of Reptibark and fir/spagnum moss mix. Ambient temps in a similar tank without a UTH and running two 25w DayGlo bulbs stays pretty much at 82 degrees and 67% humidity (day).

Adding a UTH I have a choice of the Exo Desert 8x8 8w, 10.5x11 16w, or the Rainforest 8x8 4w or the 10x11 8w.

It's for a small ball python and yes, it's temporary for now. I'd like the ambient temp to be around 85 to 88 degrees since in his tank now he stays right in the middle of the two zones 90% of the time. The temp where he likes to hang out at stays at about 85- 86 degrees.

I'm thinking of the 10.5"x11" Desert (16w) pad.

Thanks!
Jeff