The average amount of fuel burned per lap is about 1,30 laps worth of fuel. So that you dont have to worry about fuel in the race is here: Number of race laps times 1.30. Monza has 53 laps ina race (100% as an example) So you need an amount of 68,9 laps of fuel for the race. You can adjust that before the race in the tablet screen where you set up your strategy. Always take 1.3 so you have some spare fuel for rich mix.
This system is great because in real life the drivers can not switch to rich mich or something during the race. Would hurt the reliability of the engine a lot. and ers does not give enough power.