I've got a simple formula that was developed by engineers at Chrysler in the 1950s when they were working on ram induction sonic tuning. Anybody remember those big long ram tubes that cross over from one bank of cylinders to the other? Anyway:

L=72C/N

L=length of intake tract in inches
N= engine speed in rpm where maximum ram effect occurs
C=velocity of wave in ft./sec.

A good number for C is 1100 ft/sec.

However, this formula was developed using 4-stroke passenger V8 engines with relatively mild cam timing. A guess for intake timing is 265 degrees duration. For a 2-stroke, the intake duration is around 165-200 degrees, depending on the particular motor. So I modified the formula to compensate for the shorter intake duration:

L=72C(intake duration)/N(265)

When you work the math for 165 degrees of intake duration and 6500 rpm, the length comes out to 7.59 inches. This length is the total length of the intake tract from the end of the velocity stack to the face of the intake valve (reed, rotary, piston port). Using telescoping tubes and a tach would be an easy way to adjust the optimum for your motor. Just like setting a tuned pipe length - only the other end.