The final thing that needed to be done before I can get the AP to come for the CofA inspection was the compass calibration. To do this you need either a master compass or a reference line of which you know the exact heading so you can use it to then draw N/S/E/W lines and the intervals in 30/45 degrees on the ground. I used Google Earth to do this, I took a snapshot of my airport with the map setup with TRUE north up, the snapshot included the main runway and some taxiways close to my hangar. I then used a paint program to draw lines in line with the runway centre line and the lines on the taxiways. The paint program tells me the angle at which those lines are drawn which should correspond to TRUE. The angle of the line drawn on the centreline was showing 349.39 degrees true which is 336.99 magnetic (12.4E at my airport). This is 0.01 degrees off the published 337 degrees magnetic. This proves my method is accurate! I then just did the same with the lines on the taxiways to calculate their correct magnetic heading. and then I had my reference lines I could use to do the compass check/calibration, which turned out fine BTW, only very minor adjustment was needed to remain within 1 degree (well as near as I can tell on my little compass) accuracy.
