As an aside, do you make a note in the tech log when you fill the tanks full (or to tabs or some other identifiable level)? I tend to do, if nothing else to make life easier to the investigators should I have any mishaps they would be interested on.
I had never thought there was another way to do it! This concept of relying (particularly in a rental aircraft) on a series of additions and subtractions on a piece of paper (where the fuel burn might not be as per book, or the uplift might not have been 'full value') seems very subject to error. I definitely want to see a very recent known quantity the sums start from.
Even with the fuel totaliser I feel better when it has been reset to a known quantity.