Sounds like you've done some research!
You want the surface (glass) right above the UTH to be in the 90° range. He can, and eventually will, dig around in the substrate and make his way down to the bottom when he wants to. Your job is to make it safe for him when he does.
As far as the "top of the substrate" temps, I don't really worry about it much. Just make sure you don't have more bedding than is necessary to soak up messes (think less than an inch or so) and your temps will be good.
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