Jeff,
Go with Jim Thomerson's guide for starters. You may find some more differences due to altitude, as well. Nitro and oil % can also affect time per ounce, and I don't know of a rule of thumb for these that will work for all engines and conditions.
Run-time vs ounces fuel is not usually as critical for stunt fliers. Usually the tanks are a bit too big for the thirstiest conditions you'll face, and you limit run-time by underfilling it. We only need enough to complete a pattern plus several laps, and not so much we run over-time. Pretty loose limits.
Good sized syringes (e.g., 5 fl oz) let you fill the tank, then draw a measured amount back out to get a certain amount in there. Trying to presume it is empty, and fill a measured amount, is not as sure.