And it can become somewhat more complicated if the aircraft is 'tankering' fuel
I'm going back quite a few years but, as an example, I think it was Kalamati, Greece where the fuel would be circa 50% more expensive than buying fuel at a London airport thus the operator would want to carry, out of London, as much fuel as possible based on aircraft, or airfield, performance, whichever is the most limiting factor.
To carry the excess fuel that distance might cost circa 15% of the excess in fuel burn but that's still a saving of circa 35% on the cost of the excess fuel carried, if Ops, when producing the computer fight plan, have over exaggerated the payload that would cost the airline significantly, the crew would take that payload figure before fuelling up, last minute fuel top ups might delay the flight but perhaps they could perform such a fuel top up whilst waiting for the tug