Per the forum caresheet, the max basking temp is 94 degrees. You should aim for an average basking temp of about 90 degrees. To achieve this, keepers frequently need to set the thermostat regulating the heat source to a temperature that's higher than that.
In your enclosure, you'd measure the temperature using your temp gun on the glass right over the UTH and adjust your thermostat until that temp averages 90 degrees. If that means you have to set the thermostat to 95 to 100 degrees that's alright. It's not the temperature where your thermostat's probe is that matters. It's the temperature your snake has access to that matters.
Now, if you add repticarpet and two to three inches of substrate, you are correct that your snake isn't going to have practical access to that heat. So, don't add all that substrate. Thin it out until the temperature reaches your desired basking temp. When measuring the temp, be patient when making adjustments. It may take a while for your UTH to heat the repticarpet and/or thin substrate layer.