What happens if your snake pees or takes a crap or spills his water bowl onto the thermostat probe if you decide to place it inside the enclosure? Also, the snake could move the thermostat probe if it is not secure (DO NOT USE TAPE). Both could easily result in a cooked snake!
You want to have the thermostat probe outside, underneath the enclosure on the heat mat.
The thermometer probe inside, under the substrate to know what the hottest point in the enclosure is.
Modify your thermostat temp so that the hottest point inside doesn't get above what you want your hot side to be.
Some keepers have to set their thermostat to 100+ degrees to achieve the desired hot spot temperature inside under the substrate.