Is there a formula to work out the optimum header length for gas engines?

I have a YW 48 twin, i'm going to use my CHP 50cc canister with it. I am about the weld up the manifolds (2 into 1) and need to know the best length.

Does this work on the volume of the can, or something similar?

Anyone help, or give me a good starting length, i guess a length which people use for 25cc petrols (roto etc) would give me a good start.

Cheers!

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.

If youre able to braze - ,a simple can will make a muffler -here are a couple of examples .The main thing on this design is to eliminate any baffle close to the end of the head pipe . These are simply expansion chambers.
Not knowing what the can is that you have - there is no " quick" guess as to what will work
as far as the Y from the engine - make these equal length into one pipe - and use a head pipes as large in diameter as largest dimension of exhaust port (close enough )- this will eliminate restrictions at port. Most headers are a bit larger in area than the exhaust port -which is also "close enough".
As previously noted --the tuned lengths are specific to rpm /volume and type of power band desired - The common error is to read some "theory of tuned pipes " and then try to apply that data for a aerobatic or sport use powerplant
a specific tuned resonance can wreck smooth throttle response and make for a very balky engine performance.