You'll probably have to set your t stat about 10 degrees higher than your desired temp at the surface of the substrate, but like Judy said, there are some variables. Obviously, the more aspen you put in the tub, the more heat you'll need. I would recommend a temp gun over a thermometer though. Yeah, they're more money, but you can check temps in all your enclosures, and in different spots in each enclosure, quickly, without having to move the probe and wait for a reading. They're just a lot more convenient.