PLEASE turn down your thermostat right now! the SURFACE temperature should be 90F on one side and 80F on the other! You can neglect ambient temp for now.
If your temperature beneath the substrate is 120F as you said, you have to turn down the heat or else you run risk of burning your snake! Also, your heat source HAS to be plugged into a thermostat no matter what, this is very important
The acurite is much recommended by people here, I personally do not have any experiences with it. Though I do, additionally, recommend a heat gun to easily and accurately check surface temperature, they're like