Muzzy, do this.
In an unused portion of the arena, launch at 25%, and open the E6B. Set your manifold and RPM to the various power settings shown, and see what the E6B says for range and flight duration once your speed stabilizes. Finally, determine absolute minimum power setting to remain level at best glide speed (this isn't shown, you'll have to test to determine it.) Bear in mind that while in real life you couldn't use some combinations of throttle and RPM, in AH, you - at least in all the aircraft I've tested - get the best endurance by running max throttle and adjusting RPM towards minimum until you reach minimum RPM, and then reduce the throttle if you need to reduce power further.
This will give you a good sense for how much fuel you'd need, under various "levels of threat". At the 2x burn rate used in the MA, in some aircraft you need a quarter tank, some times more, to go 1 sector if you have to run flat out. In most aircraft, a quarter tank will take you 3 to 4 sectors if you can run level at absolute minimum power. So there isn't a hard and fast answer as to when you're Bingo, it's going to depend on how much time you can take getting out of Dodge.
Finally, make sure you are aware that some arena's burn fuel at different rates than others, and take that into account. The MA burns fuel as twice the "normal" rate.
<S>
