The probe needs to be on the outside of the enclosure. It prevents the snake from moving the probe, thus reading and regulating your UTH incorrectly.
Remember, a thermostat isn't a thermometer. It may not read the exact temp.
If your temp gun is reading 82-84, then set your thermostat for 96-97 degrees to give you a hot spot of 88-90. My thermostat is set to 98 to give my hot spot of 90.