I took 23laps of fuel, but ran out in lap 20....
Look guys, it's very simple: You get the trackmap plugin. You add the Fuel consumption widget. You get an average laptime. Then you take the race length (in seconds) and divide by your average laptime (in seconds). This gives you an estimate of race length in laps. You multiply that number with your average fuel consumption per lap that you get from the widget, and voila, you get your minimum fuel requirement for the race. Add to that 10-5 litres and you're set.