Ron, I've always had excellent results adjusting pipe length by simply interpreting the sensitivity of the needle valve. Start the process by setting the pipe length to something that you know is likely a bit long which should leave you with a needle valve that's rather "sloppy"....i.e. a quarter turn (or more) required between an obviously rich setting and too lean. From there, shorten the pipe in small increments until the needle becomes rather sensitive. When you reach the point where 1 or 2 clicks of the needle valve provides a sharp transition from rich to too lean, then back the pipe off about a 1/4" and you'll be real close. That should coincide with the engine not sagging in the vertical under load. This method has served me well for my Webra Race 61s and YS 60s, both short and long stroke. I've used both Webra and Hatori pipes although I much prefer Hatori. In addition, each 5% nitro increase allowed me to shorten the pipe by 1/8" with a corresponding slight increase in RPM. i.e. going from 15% to 25% fuel, shorten the pipe 1/4".

Hope this helps.