"I've always come within a minute of the half-way point on the Hawaii game. I ignore everything except flight plan time and the time we took off. We have pretty sophisticated flight planning software used by Dispatch, and the flight plan time already takes all of the other variables into account.
From experience, I know that we generally run about 2-5 minutes ahead of flight plan (although we can certainly be behind), so I subtract out a couple of minutes and divide by half, guessing a number for the seconds. I've never been very far off."