"As a result most compasses are way off, notwithstanding the deviation card carrying impressive figures like 1 degree max error".
Is this really the case? The effect from electrics in the aircraft itself aren't going to change whether you're airborne or on the ground. Provided you're using a properly calibrated site then good results are easily achieveable. A proper base should be accurate to +/- 0.2 degrees, and since I can only read my sighting compass to 1 degree and the Airpath and H.S.I in the aircraft have 5 degree increments, where are the big inaccuracies coming from - must be the pewrson doing the adjustments!