As a sailing instructor and an amateur skipper and navigator, I would definitely like something like
Navigation::MagneticDeclination.
Now, since Geo is an existing top level domain, I think Geo::MagneticDeclination would also be very good.
I think magnetic declination is better than magnetic variation (although they mean the same), because it's more clearly a technical term not subject to misunderstandings (variation may be construed as something that varies, or so).
(See also https://en.wikipedia.org/wiki/Magnetic_declination.)
