If the controller can't be calibrated, test the thermometer in boiling water and determine what temp it should boil at for your elevation and the baro pressure at that time, then either make note or adjust the thermometer if it can be.
Even if no adjustments are possible, you just need to determine what you want the thermometer to read for the temp desired.
I like to run my bottler at 187+/-1, it seems to be that close. Yours may be a wider spread, just be certain it is never below 180 nor over 190F and it should be fine.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.