Thanks for the reply Smebruce, but now I`m elevated to a higher level of confusion.
If I fly from A to B at CI 60, x kg of fuel will be burnt. If I fly the same route at CI 150 x+ kg of fuel is burnt. With regards to the second instance, the overall result is an increase in opch due extra burn (outweighs higher utilization of a/c time vs. sectors). Thus when comparing the two scenario`s the increase in fuel burn should be a large one (for your formula to work), whilst in reality, it is not. I am not doubting your explanation, I am just wondering if there is more to the formula?