That's my point Ken, nobody can make an argument using figures, we just get told it's extremely expensive to carry that extra 300kg and get regular correspondence reminding us. Over time it becomes accepted that carrying extra gas is unprofessional and expensive. But is it? How expensive is it? Nobody has ever been able to tell me because of all the reasons you stated.
I'd love it if someone with access to info such as how many sectors a year QF does could chuck in a series of assumptions and come up with some ball park figures. Anyone game?
Eg number of 737 sectors per annum x the cost of carrying CFP+ 200kg =$................