Where do you put the Thermostat probe that monitors the temps.

[scm133]

I am using a UTH attached to the side of a 10 gallon tank. I have set up a red overhead light for supplement heat. Both heating units are on the same side of the tank. I have attached both to a Repti Temp 500R Thermostat. Where is everyone placing the thermostat probe? Against the glass, on the substrate, hanging in mid air? Right now, I have the probe taped against the glass and above the substrate on the warm side. Temps are staying 81-82. I double checked the substrate with an infra red temp gun. Substrate: Warm side is 81 and cool side is 77. Is that ok? My little Pac is still doing great...eating and pooping. Thanks for any advice.