I think you are pretty close, provided the throttle is not wide open all the time, and you don't expect the thing to hover for 15 minutes...

Run an experiment...
I use a graduated beaker to check fuel draw rate... fire the engine up on a stand & run it for several minutes at different throttle settings to approximate your flying style and measure it...
3minutes 6oz... 15 minutes 30oz's

obviously this is an approximation, so you should do this and then add a 20 - 30% saftey margin or so...
(once you get flying, do a real experiment with inflight conditions)

erik