The forecast winds over the southern half of Australia were( still are?) notoriously inaccurate. One frequently used up the variable reserve (10%) westbound.
Fortunately in those days the bean counters had little sway in flt planning and experience saved many a diversion. A system developed when accurate winds were computed ie ins where crews always gave the spot wind and temp at all posn reports.
At flt planning this gave an indication of the accuracy of the forecast and fuel load determined accordingly.
From memory my biggest error was 12% over fuel burn....120 kt wind from the NW instead of from the SW.
The only time you have too much fuel is when you are on fire!
Emeritus