This is virtually impossible to calculate because there are too many variables to take in to consideration under the playing conditions in AH. While it could be estimated I supposed, I encourage people to pay attention on their own right and learn how different aircraft handle different flying conditions. Altitude, engine settings, take off weight, WEP usage, etc, they all have a bearing in AH. Ever notice how most aircraft burn fuel more efficiently as they get higher in altitude? There are a lot of aircraft that say "40 mins" when you take off and then 10 minutes later at high altitudes the "flight time" is at "38 mins" (or something similar).
Forget what the fuel gauge is telling you, get your E6B up and pay attention to that. I often hear "low on fuel, I'm RTB". I'll promptly ask "how much do you have left", and usually they reply "under a half tank", "only a 1/4th tank". My knee jerk question right after that is, "how much time do you have left, or what is your range". Inevitably, I hear "100 miles", or "13 miniutes", or some figure in that vicinity. I usually mention that they have a lot more fuel than they think and to not pay attention to the gauge, but it seems to fall on deaf ears.
The E6B tells all (or at least a far better snapshot) and it tells the picture MUCH better. Use it.
