Greetings,
Lemurian and Mutt are both right
In fact Lemurian computed the K factor using the following ,the LANDING WEIGHT DID NOT CHANGE because the increased TAKEOFF WEIGHT is PAYLOAD,
MUTT used an increased TAKEOFF WEIGHT due to EXTRAFUEL uplift, so the LANDING WIEGHT is AFFECTED and will be LOWER, which will give a LOWER K factor.
In fact the thing to do is to use MUTT result and compare the fuel price (in percentage) difference (corrected by fuel SG) to the K factor and if fuel price is greater then TAKER FUEL.
The method used by Lemurian is the one to use incase of excess weight to be offloaded. let say that for limitation purposes we have to get rid of XXXX KG , then using K we would split the weight between pay load and fuel , which in turns will save you K % of your payload which is $$$$
