Hey Oz,
You may be wanting to talk with some dispatcher guy.
It wont be an easy calculation unless we have the manuals of the aircraft, to calculate the fuel burn.
On the other hand, you can try to find on the web something like a flight planner, where you can put an average weight, and calculate for the same flight, less 288 kg, or say, less 188, if that's where you're trying to go.
Good luck.