A glimmer of hope!
I've been wrestling with my navigational problem and I think I've found a solution.
I've flown from New York to London often enough to know that when you leave JFK you head in a north easterly direction, but when you cross the shores of England you're heading south east. You follow a great circle path between the two points, and in order to follow such a circle your navigational plan has to account for an element of precession. The only question is, could you estimate how much is necessary with sufficient reliability?
Well, it may be crude, but the flight planner will give you a vector flying to your destination from the west, and it will also give you a vector for the same course in reverse.
Say I'm flying from Easter Island to Santiago, the flight planner tells me I should head in a 095 direction for 458 minutes! When I enter the reverse course it tells me I should head in a 267 direction, so there's an 8 degree difference between my departure heading and the compass point opposite my heading for the reverse course, namely 95 - 87 = 8.
I may leave Easter Island heading 095, but I should arrive at Santiago heading 087, and I should adjust my heading one degree every 458 / 8 = 57 minutes.
It's crude, but it's worth a try, and I'm feeling much more positively about the reliability of my drift meter. I hope you weren't too disappointed with the pessimism of my earlier post. I thought it kinda funny to be attempting this navigational feat with such simple instruments, and I thought I'd share.
Good luck to all.