Fuel planning
The computer flight plan that I have will give you the minimum fuel required to do the flight, it then has the minimum fuel on board at each waypoint.
So it is all quite simple, if you don't have the correct fuel quantity at a waypoint then you are going to have to divert.
I can see how this is a problem using the average fuel gauge so for most American aircraft a fuel flow meter is essential.
Fortunately the Robin I fly has four fuel tanks and by running them almost dry (a very accurate tank almost empty light is fitted) you can get a very accurate fuel plot going based on the conditioned for the day rather than filght manual numbers. I have had fuel flow numbers within a ltr after four hours flying in this aircraft however I would not run any aircraft that I did not know well without a much larger margin for error.