I am taking the temps via a thermometer probe under the substrate,, touching the glass of the tank where the heating mat sits. I'm going out today to pick up a bigger heating mat (got a bigger tank) and I'll see if that changes anything (bigger UTH, more surface area to heat, the stat probe will be less of a problem). If not I will have to look into other options, a new Tstat or something. The cool side is taken with the thermometer resting at substrate level on the cool side.