That's silly. None of the theaters/maps are going to be large enough to have any variance in magnetic declination, therefore it cannot and will not be a factor in any gameplay.
Declination would be set once when you arrive in stratis/Limnos and then forgotten about, and would most likely be done using the simple declination method anyway, that being asking someone "Hey buds, what's the declination here?"
If you're going to impliment magnetic declination, you may as well implement a "set watch to this time zone" feature.