How much extra fuel is used on an aircraft per additional pound (on your choice of aircraft and length of flight, etc.) For example, I am trying to calculate the amount of CO2 that I produce by going on a flight. I don't want to just divide the total fuel expenditure by the number of people onboard.
Of course, an extra passenger means that extra fuel might be loaded onboard to handle the weight of someone heavier than myself+my baggage and the weight of that fuel requires extra burn too, etc.
I once performed a calculation based on my best guesses and found that 'carbon offsets' that were being advertised were about 10 times higher than they should be!