I think this is right, but I hope someone else will confirm this.
OK, you need the average drift, which is 0.06 degree/hour. And we travel 6000 nm in 12 at 500kts. Then it's the 1:60 rule. Dist=Angle x Range over 60 (sorry, it's hard to type out a formula).
So we have Dist off = 0.06 x 6000nm over 60.
Which gives 6nm.
I hope this helps (if it's right!
)
[ 15 August 2001: Message edited by: Scratch One Bandit ]