I've noticed that it heavily depends on the surface the tank is on. If there is no way for the heat to escape even on a low setting the rheostat can still build up enough heat to burn. I use a dimmer that I made myself for 5 of my animals and it's vented so I am able to pretty much keep a constant temp. If it wasn't vented the heat will build up under the tank causing burns. You gotta be really careful with rheostats, especially if your room temp fluxuates. Then your gonna have a lot of fun. Also, the snakes usually burrow down to the bottom so there usually touching the bottom of the tank not the top of the substrate so recording the temp of the surface IS what you want. 115 is really hot and could cause issues so I would keep a really good eye on that.

Good Luck!