"Other than Martin's Solar method, the other way round this is to use a phone compass with GPS & set it to read true rather than compass... that works too...
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I'd be abandoning the compass pretty much, movement of the magnetic pole is stupid atm. Anywhere from 55-80km's+ a year... you'd have to keep track of the current position for non-permanent setups.
Martin's option or the phone would prove a better future proof option imo to use.
Your probably doing better than 1 degree of movement every three years using magnetic north. So, you would need the latest co-ords closest to the day you set the mount up.
I like the camera and a long exposure idea myself as well. Just mount the camera using the same rack(same position) as the telescope if possible.