In the past i have always taken temps on top of the substrate thinking it is the best way to go. But the more i think about maybe i should be somewhere in the middle in case they dig down. I have it around 87-89 degrees on top of my aspen and 93-95 or so taking the temp directly on the bottom of the tub. Either way if you have somewhere around there you're BP is going to be in good shape just wondering what most of you do. 92 on top or bottom?....