@Meta4 is right. As I’ve written about here a number of times it’s all about compass “variation” vs. ”deviation.” @solentlife correctly defines variation, however the drone calibration does not correct for variation. Only deviation. And it should be done only when there is no outside mag interference because the intent is to correct for built in interference As per @RodPad.Compass calibration has nothing to do with magnetic declination or where you are.
The only reason for compass calibration is to identify the magnetic fields which are part of the drone so they can be ignored.
Then the compass reading is accurate and not affected by any magnetic influences which are part of the drone.
The only time compass calibration is required is after rebuilding or modifying the drone.