Precisely. The whole reason for setting the probe for the thermometer against the glass is to keep track of and control the hottest temp. If it's not perfect on top of the substrate and they want to get warmer, they'll burrow. The very fact that they do this means you want the temp at the glass (or plastic) to be a safe level.
Not sure why you have that extreme difference of temps from glass to substrate top...that's strange. Have you tried placing the tank on a layer of foil to help spread the heat? Sometimes UTHs get concentrated hot spots and won't heat evenly. Just a suggestion.
But, regardless, I would get the temps on glass to no higher than 95 and just go from there. Don't worry as much about substrate temps.
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