Hi! This is the same challenge I've been facing, using about an inch of ReptiBark as substrate. The temps on the glass and up on the substrate where snakey actually is are so different. After reading more here, during the next handling session I'm planning to use less newspaper and only ¼" of bark, then stick the thermometer probe directly on the glass to measure the UTH output.

BTW, I have that same ReptiTemp 500R, and that price of $22.89 you linked to is good! Once you install it, it should prevent heat output even reaching 100.

HTH!