Is there a formula to determine if the quantity of aviation fuel that is posted as being used is correct? If I have the number of gallons, the pounds and hours flown?
The fuel for a typical GA aircraft is AvGas, 100LL (blue tint).
It weighs 6.01 lbs / gallon.
Gallons * 6.01 is the weight. For example, if you're filling up a Cessna 172SP that holds 56 gallons, it should take 336.5 lbs. of fuel.
To calculate how much has consumed, you'll have to know the burn-rate of your airplane. Again, using a 172SP, the published burn rate is 13 g/hr, but most people lean to about 10 g/hr. So you can expect that, per hour flown, the plane should use about 60 lbs. of fuel.
Of course, a plane will use more climbing, less descending, and a certain amount for ground operations. So the exact details will depend on your flight profile.