I have two measuring sticks; one for my '31 (11 gallon tank) and another for my '28 (10 gallon tank). I got my initial 11 gallon measurements from an earlier "Forum" thread as indicated by the earlier H. L. Chauvin post. However, on that 'thread' there were two (or maybe three) posts that gave conflicting measurements, so I did as reported below:
I could not at the time find a reliable scale for the 10 gallon tank, so I did what was suggested on one of the posts and drained my tank, added one gallon of gas, measured and marked a virgin paint stirring stick; added another gallon (now 2 gallon total) and again stuck the stick in, and making a '2 gallon' mark on the 'dip stick'. And so on until I reached the 10 gallon mark. This method is tedious, but assures accuracy and only need be done once in the lifetime of your ownership.
For my paint stick measurements, I both etched, notched, and marked each of the gallon increments. I then made about three duplicate sticks for future needs.