This is the FOS exhaust formula/design. Developed by Frits Overmars. This man has probably forgotten more about 2 stroke exhaust systems then most people can hope to learn. He shared his calculations and design online that he recommends as a starting point. There is no magic way to get the perfect design the first time but these numbers will get you well within the ballpark and probably outperform most stock and older aftermarket designs.
Underdogs racing has developed an easy online calculator that will spit out the design for you.
http://www.underdogsracing.com/fospipe/
Ezin Hobeki also has a program you can download as a demo that can create a FOS design chamber.
http://ypvsbox.free.fr/?cat=10&lang=en_us
Remember to include the "restrictor" at the end of the rear cone. Below is a drawing for the design to use. These can be made interchangeable and used to manipulate the powerband for specific tracks. It also has the added benefit of making the tailpipe length irrelevant which can make fitment and fabrication much easier on yourself.
If you then need turn the measurements into actual cones there are several online cone layout programs that can create templates for manual metal cutting or fit up with card stock, or export files for laser/water jets to cut. They can also help with turning right circular cones into oblique circular cones to help with clearance and fitment on certain bikes.
Cone Layout 2.0 is a good option for doing this.
https://www.conelayout.com/
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