Again, for 777.
in the introduction part of the MEL/CDL, you have a chapter called Enroute Fuel Mileage Effects. « ... increase in flight planning fuel of 0.25% per 454kg of enroute climb weight penalty... »
another: Weight Reductions « ...for each missing item ... reduce the takeoff, landing and enroute climb limits by 46 kg... »
So I guess tou can treat it as the same weight in fuel. I don’t think you’ll find a SOP for that but probably have a rule of thumb for x kg equates y ft on Rec Max (on the T7 it’s something like 100ft for a tonne).