RTB and UTH Safe Max Safe Temp Range

I know...I posted this is another section of the forums but it didn't look like it would get any attention there so here goes...

I see much talk about UTH's and belly burns on the net, but my questions is what specific temp is considered entering the "risky zone" so far as burning goes (measured at the glass inside the tank where the UTH is in contact).

Our RTB at times loves to bury under the substrate and lays directly on the glass. He has two low temp UTH's (Exo Tropicals) to assist in preventing regurg (which it has) but want to put a higher temp Exo Desert UTH on the one end for him and remove the other low temp UTH.

So measured at glass surface contact, what is the upper safe temp range a UTH should get?

Thanks for the input!

Not sure what the upper temp range can get to but why risk getting to the upper temp in the first place? Put a thermostat on your UTHs if you do not have one and then monitor the top of the glass with a thermometer untill you get to you 90-92 range, just check it during the day and adjust the UTHs untill it gets to this range.