From some first principles and a bit of Googled data.
Fuel is burned to overcome Total Drag, of which
Lift Induced Drag is,say, nominally 30% of Total Drag.
So, 30% of fuel burn is proportional to AUW.
Herc fuel burn (guessing) 5,500 lbs/hr?
Sector time roughly 2:30hrs
Total fuel burn 13,750 lbs
30% of this is 4125 lbs.
Nominal AUW, say, 165,000 lbs
So variable Fuel burn per lb of AUW = 4125/165,000 = 0.025
So Delta fuel burn for 2.5 tonnes (5,500 lbs) = 5,500 x 0.025 = 137.5 lbs
Different route but same ball park as Larssnowpharter above - must be right then!!!