According to my 1986 O'Day 222 owner's manual the length of the Jib sheet is 35' X 3/8" Tro-Blue braid. I don't remember the Jib sheet as one continuous line to be tied to the clew of the Genoa. If it did come that way I would have cut the line in half and made two separate sheets out of it.
I think that 16.5' will put you in the ball park because I have a superfluous length on both of my sheets.