In fact I think that the real problem is not a calculator or another .... it's the way the driver is using it mostly.
The right amount of necessary fuel may only be calculated on a precise correct base: how many liters fuel are needed for one lap.
Mostly we, as usual drivers, calculate it after a few laps alone on track during solo runs when preparing a future online or offline race..... that's finally the moment when the mistake is done at the start ! ....
Compare a few quick laps alone on track at full speed without any other driver on track ..... and a future race of 20 laps with 20 other drivers .... that's not the same: the fuel consumption per lap will be rather different and mostly exagerated for the few quick laps alone.... ( and mostly we do that with much less fuel than will be needed for the start of the race !! )
Run 20 laps with a full tank with other cars on track .... see the remaining fuel amount at the end .... to know how many liters were used ... divided by 20 laps and ... it should then be the +- right value.
The example you gave .... is often what happens .... whatever the calculator you chose.
If the base ( liters per lap ) is unprecise or not correctly calculated ..... due mostly to not enough laps for it .... all calculators will show the same problem ( even if you're your own calculator
.... I know it as I often did the same at the beginning ).
