When setting up suspension how do you account for the huge weight difference of a full tank vs. Say a tank thats just a quarter full?
Do your measurements on a half full tank and don't worry about full or near empty.
By the time you account for rider + 919 bike, the fuel weight variance is not that large an factor.
Look at it this way, suppose you go up 7 litres from half or down 7 litres from half.
7 litres of gasoline is roughly 11 pounds or 5 kg.
11 pounds on 640/650 pounds is not even a 2 % change.
Suppose you had 0.95 front springs.
A change in bike weight of 11 pounds total might change your front end sag by 1 mm at the front and 1.6 mm at the back.
The idea is to find the best overall spring rate and get the preload right.