Hi Dan,
If you have an Hatori 605 pipe (short stroke pipe) I'd start at 14.5" to the first baffle if you plan to run a high rpm prop (11x7 or 11x8). If you wan to run a larger prop (11x11, 12x10) then I'd pull the pipe out to 16.5". As long as the engine is rich enough and the pipe is long enough then you will maintain rpm in sustained verticals. If not (and the engine is not going lean) then pull the pipe out another 1/2".
Good luck,
Will