Rusty,
I just gotta ask - why don't you just copy Tracy's intake and be done with it???  

I calculated intake runner length for a PP engine last year but unfortunately I don't have the results here in sunny Florida (playing tourist, much to the chagrin of the natives).  So these numbers mean nothing, but I THINK that I calculated that a 16" runner length for 7400 RPM (2.85 redrive) and 24" for either 5600 or 6000 RPM.  The problem we are all seeing here (and Ed is addressing) is that we need several distinct lengths to correspond to the differing engine RPM.  Since I am dealing with a PP engine I am sorta on my on but Ed's work has been a great help because his formulas will allow you to calculate the length for a given RPM.  My thought on the solution to this is a little different from Ed's infinitely variable length but simpler to build - since a PP engine only needs 2 runners (1 per rotor) the idea is to have a short runner (16") for high rpm and a long runner (24") for cruise; a butterfly valve will be used to switch between the two.  I hope that this setup will not need to be automated - set the butterfly to 'long' for takeoff, climb and low altitude cruise and 'short' for WFO and high altitude cruise.  

Ken P.