GtW, I know this is nothing to do with the main thread but, I had a chat with somebody who knows about such stuff and it turns out that your DF fix was well outside the required tolerance of the kit at D&D - it's being looked into.
Ah! another good reason for practice pans
On the only occasion my instructor did one ,some years ago, he made a point of reporting our position (a couple of miles different), probably to make the point we were not lost but it did also confirm the system was working to spec.