I just received a Gossen pilot light meter which I bought on EBay. It is very easy to learn to use except for one little screw behind the light meter which says "zero Setting". From that, I assume it is a screw you turn to calibrate the meter. so, what I would like to know is: What exactly does the "zero setting" do and if it is indeed meant for calibrating the meter, How would I go about it? As always, Thanks!