Thanks to all of you for the advice. I am going to try it in the air with the GPS and keep flying the points of the compass until it is right. Problem with doing it on the ground is that it should be done with the engine running, but the aircraft is an RV6 taildragger, so not possible to put up on tressles to the flying position !! I did it that way many years ago in a Taildragger Robin DR221 but had forgotten the proceedure until you all reminded me --- thanks