Any good words on the best way to box (calibrate) the compass? What is the best thing to use for detecting NSEW (GPS?) against which to make adjustments. What tolerance is good enough? If it makes any difference, I just installed a new Danforth compass with both a NS and EW adjustment. It took me quite a while to get it right (if I got it right), using the GPS. I could get the GPS and the compass reliably to within a degree or two, but never on the mark in eight directions.