Here is a half baked thought.

Look at using an ADC input.
60 to 185. Scale this by 60 giving 1 to ~3 for the input. Set VREF for 4 volts. Now you have a 1 volt buffer either way for when the plate suddenly warps or something else happens.

Now the optimal voltage can be set as VAR SetPoint and the hysteresis can now be SetPoint +- what ever. Beyond the +- all stop or torch out or...

Have the analog at 10 bit giving 1024 steps in the 4 volt range I would think would allow for a nice smooth operation.