The only variable is in the burn factor. Memories fade but I think I used to use 3.5 for 73NG and 4.5 for Classic. It also all depends on your company 'telling' you what they actually pay for fuel where, and that can sometimes be prized information! It is very easy to write a spreadsheet to determine tanking taking in all factors - next sector fuel, MLW. next MTOW, fuel costs, tank capacity, mother's maiden name etc etc. once you know the factor and the fuel costs.