[Christ]

I may be incorrect about this, but I believe header primary tube length is measured as the length of the pipe before a sudden change in volume. If you have stepped headers, the "primary" is much shorter than the entire header pipe.

Helmholtz frequency tuning is (again, IIRC) done taking volume changes into account, because each volume change creates a reflection of the sonic frequency in the exhaust.

In intake tuning (which is somewhat close to similar, so I'll mention it here), the intake "runner length" is measured as the distance from the face of the intake valve to the plenum opening, which is a sudden volume change.

Helmholtz tuning doesn't have to be ideal to be effective (I pointed this out earlier), but in a non-ideal situation, efficiency is lost to energy depletion from excess wave travel.

Frequency tuning is done on a system of halves and doubles, on (normally) 4 orders of frequency. If your ideal area is at 10,000 RPM, there are 3 half frequencies and 3 double frequencies, each one with a doubled loss percentage from depleted energy. The halves would be at 5,000, 2,500, and 1,250 RPM, while the doubles would be beyond the reach of most any engine we're interested in.

Notice where the 3rd frequency is? 2,500~... That means that even if you're tuned for the best harmonics at 10,000, you'll still get a gain at 2,500, at a benefit loss ~10-12% or so. 5,000 would be the 3-5% loss that Old Tele Man referred to, and 1,250 RPM would be ~20-25% loss.

Don't focus on the loss, though. It's still "free" energy that would otherwise be wasted.