There is a relationship between rpm, engine capacity port timing, header length and caninster first expansion chamber volume. Add som flow influencing parameters as well.
If it was easily calculated in a general formula, all canister producers would have top-ranking products.
After the muffler is made, the only variable available to the experimenter is header length, so on the test stand, you start with a very long header, and work your way down in length until satisfied. Then you find a way to accomodate that length in the limited space available outside, or inside the canister.