It works on a knock-out system. In Q1 all the cars try to go fastest. After that session ends the slowest 5 cars get knocked out. Leaving 15 cars.
The 15 cars again race to go fastest (this is usually on fumes as well). The slowest 5 then get taken out. The remaining 10 cars have to qualify on race-fuel. They start the race with the fuel they have at the end of Q3. P11-20 can fill as much as they like.
Hope this helps
"It could be done manually. It would take quite a while, but it could be done. There is however a much more efficient and accurate way of getting the data. Men with lasers." Wing Commander Andy Green