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.
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