I recently switched substrate to aspen for my natures spirit rack. I use a johnson thermostat with the probe taped to the flexwatt. The rack I use has 3 levels and holds 3 41 qt sterilite tubs. Basically what I want to know is, if you have a similar setup or use the same t-stat as I do what do you keep your thermostat at to the get the temps right above the aspen?