go to home depot or canadian tire and buy a Dimmer switch, you can plug your heat cable into it, and then you'll have to use trial and error to find out how to get your cable to the right temperature.
It seems you have the thermometer probe in the right spot (above the hot spot under the substrate), leave it there and use it to measure how hot your heat cable is getting. Remember that if you have thick wooden bedding (aspen or anything that isnt paper towel or newspaper), it takes much longer to heat it up and get it to the right temps. Also, you should only have a little less than a 1/2" of bedding above the hot spot, and your snake will burrow if he feels he wants to be warmer.
The hottest I usually leave my heat pad is 95, and whenever I see it at 95 I dial my thermostat down until it goes to around 92~. I like it when it sits between 92 and 93. And everytime Ive taken her out she always feels pretty warm, but not TOO warm.
Watch this video:
Snipers Burn
- YouTube
It shows the aftermath of a snake getting out of its cage and sitting on top of the actual heating element. The burn looks terrible, but proper aftercare was taken with antibiotics. You should at least check your snake's belly every so often to make sure there are no burns, if you cant afford a thermostat. If the ambient temps are cold, its likely that a snake will hang out on the hotspot way too long, even when its too hot, because the air ontop of them feels cold.