Hey Leon,
I acknowledge this is wikeedia but, I can provide more references if you would like.
https://en.m.wikipedia.org/wiki/Magnetic_declination
Have a read, specifically under the heading ' using declination' specifically with respect to the non adjustable compass
Magnetic declination & variation are one & the same. A magnetic compass is reference to the north magnetic pole (red tip always points magnetic north) so it's irrelevant whether you are in the northern or southern hemisphere. Variation is applied uniformly in relation to magnetic north irrespective of where you are. Easterly variation is added to the compass bearing, westerly variation is subtracted.
This link explains the CADET Nmeumonic Compass to True & reverse for true to magnetic.
https://www.getlostpowerboattraining...alculate-them/
Ignore the reference to deviation, this refers to the magnetic field around a fixed compass generated by steel fittings of a ship for example. This is measurable & is corrected out using magnets undertaking a process known as a compass swing.
Hope this helps