Aha, the trick, JMR, is I plot my last waypoint at the mouth of the harbor. It gives Python a reference point; all the saves of the game thereafter, supply's Python with the geographic codes (numbers that are divisible by 120,000)(don't ask me why it just does) wherever your boat is located. Its up to you the player to zero in with correct sight reductions to plot your location.
I'll show you a plot I'm doing in my present SH3 patrol as soon as I get it copied to imageshack.
edit:
Ok, here's a 3 star fix just outside of Lorient with attendant sight reductions.
cheers,