Race duration, lap time, fuel use — done. Counts ACC's extra lap in automatically and gives you an honest reserve.
In ACC the last lap starts as long as the clock is still running — in a 20-minute race you will therefore almost always drive one lap more than 20 ÷ lap time. The calculator adds this extra lap automatically.
The recommendation puts one more lap of reserve on top: lap times vary, fights cost fuel, and an out lap at the start uses some too. One lap too much costs a few kilos of weight — rolling to a stop empty costs the race.
Coasting to a stop on the last lap is the most annoying way to lose a race. This calculator takes race duration, your lap time and your fuel use per lap and works out how much you have to load — including the ACC quirk that the last lap still starts as long as the clock is running, so you almost always drive one lap more.
On top of that, one lap of reserve is recommended: lap times vary, and fighting costs fuel. One lap too much only costs a few kilos of weight — one lap too little costs the race. ACC shows your own fuel use per lap in the MFD on the fuel page — that is the only reliable source, because it depends on car, track, engine map and driving style. As a rough ballpark, GT3 cars sit at about 2.5–4.0 liters per lap on the normal circuits. The Nordschleife is the big exception: at a good 25 km of lap length it uses a multiple of that.
Laps = race duration divided by lap time, rounded up, plus one extra lap (the ACC timer still lets the last lap start). That times your fuel use per lap, plus one lap of reserve. This is exactly what the calculator works out automatically.
In the MFD (the multifunction display, cycled through in the cockpit with the arrow keys) on the fuel page — it shows the consumption of the last lap. Rule of thumb for GT3: 2.5 to 3.5 liters per lap depending on the track.
Because in ACC the last lap still begins as long as the race clock is running. In a 20-minute race you therefore almost always drive one lap more than plain division gives you — forget it and you run dry on the last lap.