I'm getting my manz a new set up tonight, and I've seen people put the probe in-between the plastic and heat mat, or on top of the plastic in the substrate. Which one is better?
The probe should always be sandwiched between the outside of the bottom of the enclosure and the heat mat as shown in this image from Jmcrook's link:
You don't want the probe inside the enclosure under the substrate because the snake can move it or pee on it which will cause inaccurate readings and lead to improper temperatures.
Instead what you want to do is use an infrared thermometer gun like this to measure the temperature of the floor of the inside of the enclosure (underneath the substrate so you are accurately measuring the hottest temperature as snakes may burrow down through the substrate to the floor). Then you adjust the thermostat so that you get the right temperature with the infrared thermometer (88-90F) on the floor of the enclosure.