Old topic kicked up...
In case you have a double FMC failure when crossing the Atlantic, on a 737 NG with no CPDLC etc., and your flight plan data is something as:
5130N average true track 073
5220N average true track 081
And your FMC's fail at 25W. Which course do you fly?
Apparently, it's 083 true. I just don't know the correct procedure to get to that.