Yep that's what I'm looking at.
Having slept on the problem, I've a feeling that the published graph includes oil burn as well as fuel. My spreadsheet version starts at the correct full fuel position (corresponding to 18 gallon) but ends 'high'. The only way I can see why this would be is that something else forward of the CoG is disappearing and I can only guess that is oil.
I'll calculate the start and finish positions as a cross check and see what I can come up with.
Many thanks for the help.