In my RBI rack my thermostat is set at about 103 degrees to get 90 degree temps in the tubs. I know you're aware but be sure to remove the tape before placing any snakes in there.

Now, why are you fumbling around with thermometers and where to put the probe and obsessing about this if you have a temp gun? Lol no offense but I'd think depending on what type of temp gun you have it'll be much more accurate than most cheap digital thermometers. Just set the thermostat to 95 and test the temps in the tubs after a few hours with your temp gun, and adjust as needed. Once you find the proper temp to keep the thermostat at you shouldn't need to adjust it that often, if at all. Just check the temps with your temp gun every few days