Spider robotics probably will have some useful instructions on setting up the herpstat; I'm sure you can set day and night temps. It should also include placement of the probe. But from my understanding the probe is place on one of the heat tape sections where the snake is housed. You should still use a temperature reader on the interior of the tank where the snake is housed.
I would suggest this because the plastic bin will absorb some of the energy emitted from the heat tape. Therefore the temperature on the tape might read 95 and the temp inside the bin on the same spot might read 93. These aren't exact number, I'm just theorizing.