Trip fuel vs zfw is not a linear relationship
Mand fuel vs zfw is not a linear relationship.
ie if the zfw is down 2% then the burn is not down 2%....worse if the zfw is increased. Why ? Too long a discussion.
Within small variance in ZFW then the corrections are relatively accurate and I would suggest to a limit of approx 2T ULH and 5T on short haul flights.
Within those boundaries I do an adjustment and add a bit for inaccuracies so that I am on the "right" side of the error.
Outside that then get a new flight plan.