I'll probably get shot down for being so unscientific, but I think Lindberg invented it so blame him...
Assuming that my route is all more or less on the same vector, plan my first bit of the route (at cruising altitude and speed) flying overhead two visible landmarks. Flying from one to the other, take the compass heading, compare to the true track.
Then for the rest of that route (or until I've turned more than about 30° in which case I'll repeat the exercise) take that difference as a fixed difference between magnetic heading and true track.
G