It entirely depends on how you're driving it, not just the number of miles, but whether you're going over the windy, hilly bits and how spirited you'll be.
I'd work it out at your normal mpg going motorways all the way, then add a couple of tanks full for diversions up and down mountains just to be safe...actually, maybe three. But then again, I was on the windy bits all the way from Lyons last time I did it.