It would have been great if the 10ltr stages of filling had coincided with a consistant amounts of graduation marks at each increase of 10ltrs but this was not the case.
I'm sure the a/c owners would be happy to assist in this project when they realise the benefits of what you hope to acheive. Perhaps the offer of a "gauge for the plane" as well as your personal gauge would be a way forward!
Good luck.