At what temperature should I set my thermostat?

Got my snake 3 weeks ago I just wanted to know how much higher should I set my thermostat to make the temp on the high side hide about 88-90? Without so called the bottom of the tank getting to high an I mean through the substrate... because I’m sure u have to set it higher so the heat can get through

Ok, you want to be using an infrared temp gun to measure the actual surface of the enclosure. If you're using glass, measure the glass, NOT the substrate.
The reason being: snakes burrow and will move substrate around. So, if you measure the top of the substrate and your snake burrows, the glass will be too hot. If your snake wants the warmer temps, they know how to dig for them.

As for what temp to set the t-stat to, that will be different for every enclosure. It just takes some trial and error. Start a few degrees above your desired temp and work your way up. To achieve a hot spot of 89-90 you may need to set your tstat at 92, or as high as 96 or 97.

At whatever temps it will allow you to reach optimum temp in your enclosure, everyone setup is different depends on enclosure, substrate etc

You have to play around with it.