Even today they do long runs in F1 to check fuel load and tires. And I remember that even today it might be possible to have not enough fuel regardless how much computer power is used to not have to much fuel

To short this tries sometimes I checked for half of the race distance when I had not enough practise time before a race started.
The reason why the numbers never had been accurate was that track length plays a rule as well as your driving line. And if I'm not totally wrong there are 2 track lengths. I would say the one length is the visible to the car track length while the other is the invisible trk length.
If it shows a lap number of 25 rather then 28 t makes you wonder if fuel fill be enough. The original was the other way around which is worse because you meant to have enough fuel while in sim reality you run out of fuel. Still, you will only notice when you actually try this.
A good test is to start GPL, select multi player, start a host, adjust AI members (a low or none), adjust difficulty and adjust qualify/racetime. For 28 laps spa I would try with 14 laps qualify. 45 - 60 minutes should allow you to run one complete 14 laps stint. My way was to fill in my fuel for 14 laps and run the car until dry. When run dry check the number of timed laps, add one and you have the number of laps. For a 28 lap race multiply by 2

For int short races I preferred to run full stints because they helped me getting used to car with full tanks until race end and they helped me learning the tracks.
Just my 2 cents
