Great circle formulae, True cource and actual heading

"Sims" writes:

I guess i was looking for a formula to calculate my True Course +Magnetic
variation at any given point on the globe.

Formulas can tell you the true course needed at all points along the
great circle route, because it's just geometry. But the local magnetic
variation is not predictable by a formula. Just look at a map of lines
of constant variation - they're too irregular (and they move with time).

The best you can do is a table plus interpolation, or a set of functions
empirically fitted to measured data. Either way, it won't be just a
simple formula.

On the other hand, even if you knew the magnetic course you need to
follow exactly, that still wouldn't tell you the heading to fly or
sail, since the heading also needs to provide wind correction.

If you have a GPS along on your trip, it can show you whether you are to
the left or right of the ideal great-circle path between two waypoints.
The correct heading is whatever heading keeps you along the centerline
of that path. There's no better definition of correct heading.

Dave