CortaVento,
Further to my previous suggestion, some actual figures for a 5:30 flight just completed -
Additional fuel per 1000 Kg ZFW increase = 227 Kg
That's 4.3% per 1000 Kg per hour, pretty close to the earlier 4% suggestions.
Bear in mind that that's for a B777-200 with RR Trent 892 engines, it will vary for the B777-300 and for other engines. Bear in mind too, that the flight I just operated was a positioning flight at just 206.3 Tonnes TOW, and will obviously be higher at more typical operating weights, or on shorter sectors where Climb fuel forms a much larger proportion of the overall burn. (The opposite for longer sectors).
Also, for info, the adjustment for a 1000 Kg DECREASE in ZFW was 199 Kg, i.e. -3.75% / Tonne / Hour. All of which approximates to 4% "Ball Park".
Regards,
Old Smokey