I had forgotten that when I first had the installation, there were discrepancies in the heading. I got the installing company, in another town, send me the installation programming card, and I went out and swang the compass. I didn't just do it once. It turns out that if you get one right on with the indications on your compass correction card, the other directions are off. I kept doing it until I had every primary direction within 2 degrees of the proper indication.
Then I took the aircraft up for a flight, and double checked everything by flying rectangles, and checking that the wind indicator consistently showed the wind from exactly the same direction and speed. Once I got this straight, I sent them back their card, and the plane has been exactly correct every flight since.