I have recently acquired a Mitchell Fotoval PM-17. From what I have found online, I think I understand how to use it to determine paper exposure time and paper grade. However, a recurring statement is that the meter needs to be calibrated first. Does anybody have any information about the calibration of a darkroom light meter? I may be able to get some value even from the instructions for different models or makes, so if you have experience with them, I will greatly appreciate your advice. In particular, there are four adjustment knobs on the meter (see this picture) that seem to be intended to calibrate the meter, but I am not sure exactly how. Again, any suggestions are appreciated! Thank you!