I have my UTH's on the outside bottom of my tank, one on the hot side and one on the cool side. I check the temps on the inside surface floor below the substrate. The hot side shows 89-90 and the cool side is on 80. I then cover it back up with substrate. Sometimes he burrows down to the glass surface sometimes he doesn't. I wouldn't try to get the substrate to 90 degrees, because as you have shown, the floor temps are way to high to achieve the temps when measured on top of the substrate.
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