Check it this way...
At the fuel dock fill it until the overflow disperses fuel (with a containment bottle of some kind).Get a reading on the hour meter.Go out and run the engine for at least 10 to 15 hours. Preferably at cruising speeds.Come back to fuel, repeat the same procedure. Check the Gallons per Hour used. This is a great way to judge your "personal" gallons per hour, and give you an idea of your cruising range. I kept a log of fuelings and tried to fuel at 3/4 tank 1/2 tank and 1/4 tank guage readings. Now when I look at the gauge and hour meter I can mentally see how much has been used. This worked for me and was easy to do.Keep a log of everything you do aboard. When you review it, many things can be learned.