Hi

Measure the temperature right at the thermistor and I'd bet you get 80C +/- 5C. Measure it on the top of the glass and you will *always* get something colder. You have heat loss (thermal resistance) through the heater PCB and through the glass. A difference of 20 to 30 degrees is pretty typical. It's an issue with all similar heated beds so it's already "in the numbers" that you see posted. If you build a bed that actually *does* monitor surface temperature, you have to re-calibrate all the posted numbers so they match your (unusual) bed. Yes, I have a printer like that and going back and forth is a bit of a pain.

Bob