Ziggy the author himself says that the 1700 figure is a little high on the gas velocity, and remember that the author is talking about a full sized 2 stroke gasoline engine. Chamber temperatures and therefore exhaust temps are a lot higher in these motors than ours with the lower flashpoint. Lower exhaust temps = slower motion of the wave through the gas. I suspect that the calculation is not far wrong, except that the 1700 is wrong on our motors. Perhaps reverse engineer some calculations based on the dyno results you have available and see if you can find a more realistic constant. I have done just that and arrived at a constant that actually seems to work fairly well.

Also I am sure that you know this, but in 2 chamber pipes the convergent cone is not at the end of the pipe, it is internally and is perforated with holes to allow the gasses into the 2nd chamber and out the stinger. This is somewhat different to a regular motorcycle pipe. You must measure the pipe length from the exhaust port to halfway down the complete convergent cone using the above formula.