It would be nice if DJI engineering would pop in an tell us stuff. The best we have is people like Ianwood and Blade Strike who seem to be one removed from the internals at DJI but a hell of a lot closer than the rest of us. If you haven't, read Ianwood's magnum opus on compass calibrations.
Dji has, in their manual and videos, they suggest calibrating before each flight
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