This is sort of related I guess... I've been frustrating over the temps in my tote and finally hit on the fact that the prob I had stuck to the bottom right over the heat mat was reading like 98 degrees, but when I put another prob in the exact same spot only on top of the two pieces of paper towel it read 94. So the thickness of the paper (or in my case paper towel) really does effect the amount of heat getting through.
Also, my thermometer on the cool side was reading about 75 and thats what had me so worried, but I realized it was reading the air temp about 8 inches above the bottom of the tank, when I took a reading at the bottom of the tank on the cool side it was actually 84 degrees. I know an almost 10 degree difference sounds crazy in just 8 inches difference but I've tried a few different thermometers and they all come out the same.