Okay, let me rephrase my problem.

I am measuring temps a few different ways.
1. I have a thermometer with a probe
2. I have a heat gun
3. I have a thermostat

All of these devices are reading temps that are more than 5 degrees off from eachother. Which one should I use as a constant. Everyone is saying the heat gun but I keep getting such strange readings with it. I take a reading from the same spot and it says 100, then 98, then 95, then 100 again. That is not good. That is too much of a difference to know the actual temp.

I need to know which device I should trust to measure my temps. This has nothing to do with probe placement, I have my thermostat probe on the outside of the tubs and that is that.