Ok so figured it out
Reason I was stuck before is because I was trying to work it out but wasn't considering that some flights weren't full depending on whether demand had already been met on previous flights that day (either bus/1st/cargo were empty). I also didn't consider that the weight distribution is different for Economy, Business & First Class.
When looking at fuel consumption it says L/100km/pax, this is only correct for Economy Pax. Instead it should probably say L/100km/100kg instead with the following weight distribution:
1 Economy Pax = 100kg (or 1 original pax weight unit)
1 Business Pax = 125kg (1.25 pax weight uni
1 First Class Pax = 150kg (1.5 pax weight uni
1 Tonnes of Cargo = 1,000kg (10 pax weight units)
So the Liters of Fuel for a flight = Weight in 100s of kg * Distance in 100 km's
And finally the total fuel cost = Liters of Fuel * Fuel Price (Currently 93.75 cents per Liter)
Not sure if the fuel price fluctuates