The numbers are reasonably accurate, but it's much nicer to be able to flip the lever down without thinking, and have it be the right length. Later models do this by having a set screw either in the lever or beside it.
Sporter, if I were you I would probably tape a paper clip right below the lever when it was at the length I wanted. You would have to move the paper clip when you wanted to change the stitch length, but it would work.