Just realised my calculations were wrong, I converted lbs into N and it should have been kg
So,
Max Take off Weight = 21tonnes
Fuel Used = 2,200lbs = 1tonne
Therefore lift generated after 1 tonne of fuel used =
20-1 = 20tonnes = 20 x 1000 = 20,000kg
20,000 x 9.81 = 196200N = 196.2kN
Thats the only way I can think of doing it?
Cheers