I *highly* recommend purchasing a temp gun if you want precise control over your temps.

Put the probe in a convenient safe location close to the hot spot (make sure that the reptile can't accidentally dislodge it) and set the thermostat to the temp that you want. After a few hours, point the temp gun at the hot spot and adjust your temp accordingly. In other words, don't pay too much attention to the temp reading on the thermostat. Use your temp gun on the hot spot and then adjust the thermostat to get the temp that you want. Better yet, point the temp gun directly on the reptile when it's in the hot spot because it retains heat differently than substrate.

For example, even though my herpstat is set at 82 and the probe is on the hot spot, the temp on my chondro is 81 when he's not directly under it. If he were, then his temp would be around 86.