I place the stat probe sensor side down on a black strip in the center of the heat mat. The digital thermometer probe is on the substrate and shows that the temps are spot on, hot and cool end. (heat mat is outside of the tubs)
Only reason I put the mat stat directly onto the heat mat is because my snakes very often burrow and I wouldn't wanna risk the bottom of the tubs which is over the mat being overly hot because the mat sensor is only picking up the temp on top of the substrate.