Thanks for the responses, guys. Mark, I hear you about the wiper interference. My compass "might" be immune from this as I have it mounted on a homemade aluminum bracket, whereby it hangs slightly "out-from" and below the bottom of the dashboard; directly under the steering wheel. It's quite a distance from that motor.
ATGEP, I suspect you might be right about that "compromise".....(if I read you correctly) meaning, I probably won't get absolute accurate readings in all four quadrants. It'll be interesting to see how close I can get.
Bean, before mounting the compass, I might have done something like you suggested. What I did was, tape the compass to a small board, then I clamped another board to a table. That enabled me to slide the parallel boards together (with the compass on zero) and then turn 180. Amazingly, it was very close. Leads me to believe there may have been some corrections done at the factory. I'm not sure I quite understand why you don't think it's possible to correct on the water?? Quite often, the conditions in Ipswich Bay are as flat and calm as can be....especially early in the morning. (What might help is, my rig seems to hold a steady course without even touching the wheel). Also, I'm thinking there's got to be at least "some" deviation being created, when running, that wouldn't appear when the boat is stopped and sitting still. To me, it seems things like the tach or sounder could have an effect on the adjustments. For that reason, I thought corrections under real operating conditions might be the way to go. We shall see, I guess.
Thanks again, Yat