They will most likely burrow under the substrate anyways, so make sure that probe is hot glued down on the inside of the tank to the bottom center of the hotspot. Now, the thermostat is set at 92...but you may have to set it at a higher degree to achieve 90-92 inside the tank. I have my herpstat set to 106 in my rack to achieve 92 inside the rack...see what I mean? I'd raise it a few degrees and then check the thermometer, do some trial and error before you find what works for you.

As far as the substrate...IMO 1.5" of coco is WAY too much. 1/2" is about all you need. As long as the hotspot on the glass is only 92 max, it will not be a safety issue, however with too much substrate the heat will not get through at all.