I know this might be irrelevant but did you use a fuel planner
If not I have a good one it’s http://fuelplanner.com/
All you do is put the plane you intend on using, the departure airport and the arrival in ICAO (like YSSY or KLAX for example)
Make sure the units are metric Kgs or imperial lbs (depending on your weight units on your settings)
Also select it if your either going FAR domestic or international
If all of that’s done press planner
Then copy the “fuel on board”
Hopefully this helps
Edit: Johannesburg’s ICAO doesn’t work for some reason