For now I just moved the Reptitemp probe inside the tank, next to the hot-side thermometer probe. Both are not touching the glass, but slightly buried in the substrate. The substrate is probably about an inch deep, by the way. If that placement works out, perhaps I'll affix it more permanently, but now I've gotta get to work. I'll report back later tonight, but anyone else can feel free to chime in with their temp gradient opinions!