Hence my suggestion of "worst-case time to bingo fuel". If my Skyranger is patrolling, and worst-case to bingo is five hours, I know it can stay on station for about 10 hours.
If my Interceptor is tailing a UFO and reporting worst-case 10 minutes, I know it'll bingo in probably between 10 and 20 minutes.
The really hard part is how to deal with the fact that not only does consumption vary with speed, but also bingo-time decreases as distance from home base increases. The worst-case bingo fuel calculation should make the assumption that the craft is travelling in a straight line away from home.
Implemented this way, the bingo timer would be wildly inaccurate on a freshly launched craft, but would be very accurate once it was running on fumes. Fortunately, I only really care about the bingo timer when it's about to run out.