I would say that trying to use the analog thermometer like that is not giving you the correct temperature. It is made to measure the temp of the air, not surfaces. I would say that if it is saying the hotspot is 91 it is actually much higher. I would reduce the dimmer to drop the temp. When I got a temp gun I found the numbers I thought I was providing were actually off. A temp gun really took my keeping to the next level and much more accurate.

You can also get temp guns at the hardware store but they may be more expensive than the el cheapo ones online.

The hotspot should be large enough for the snake to get its entire body over it.