Carol,

Imagine a piece of thin steel - the steel strapping used on lumber or pallets would work great. You simply attach, say, 6 inches of that to the edge of your groundboard with a couple of screws.

Then attach your pointer to a small magnet, and stick it to the center of the steel strip.

When you need to tweak your calibration you simply slide the pointer along this strip of steel strapping until it matches the computer's azimuth setting and you are good to go.

Let me know if that is clear as mud and I will try again.

Rod