Pic. Of pot

Wouldn't it be simpler to turn the dimmer fully up? Then measure temperature of the melting pot, and when it reaches the set point, simply turn the dimmer down until the outlet voltage goes away. Mark the dimmer, and check to see if that set point holds the temp you wanted.
I've done this to set temperature devices, and it's the same method we used to calibrate oven or temp controls. Except on them we input a milliamp signal, and then turn the calibration until it's at the corresponding temperature for a certain milliamp reading at the controller.