ACCURATE ??
For rough estimate, for that length of flight, use 5 tons per hour.
For the flight, guesstimate great circle distance + 5 %,
estimate True Airspeed 480 kts, for the westbound flight estimate 50 kts headwind, for the eastbound flight estimate 50 kts tailwind.
Anything more accurate than that requires real flight planning equipment.
Oh, and don't forget that this will just give tripfuel, don't forget the additions to get blockfuel.