Ah .... in a high-tech world how quickly we forget the basics!
This is a good solution for any vehicle mounted compass (old army trick - good for planes, cars, armoured vehicles!):
Mount your compass wherever you decide. Then move out into an open field etc away from powerlines (don't forget to beware of underground pipes for sprinklers/gas etc) and get a mate to stand at LEAST 30m away and shoot a bearing on him. Note it down.
Next dismount your compass and walk over to your mate and shoot the back-bearing - this avoids introducing another compass with another possible compass error!
Then it's just a matter of comparing the vehicle-mounted compass bearing with the non-metal influenced back-bearing (ie subtract 180deg from back-bearing).
That will give you a quite accurate indication of the magnetic influence induced by the location of your compass on the vehicle!
Repeat as often as you change bike configuration and do it twice - engine on versus engine off!
P.S. Yes I also use a GPS, but I only switch it on AFTER I've come up with my location using map, compass and odometer - use it to check. As for vibration - I don't actually mount the compass directly on the bike - keep it in chest pocket and stop to pull it out if needed (still do the above procedure though).
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