The basic formulas were established ...
... by the fellow who taught leader furling to me. It is my recollection that he experimented with the actual performance of leaders made with the same material but subjected to various numbers of twist, without regard to the reduction factor. He simply adopted the number of twists for each material and starting length that he thought performed best as his suggested formula for that material in a given length. He also provided information on variations in the number of twists of the same material and length and his observations on the relative performance compared to what he considered optimum performance. Good enough for me.
Originally Posted by aged_sage
When I started experimenting on my own with a couple other materials, I set my own formulas which were based on my observation of what looked like optimum twist and what performed well more than any other consideration. Maybe not very scientific, but pragmatic and definitely effective.
I didn't hear / know anything about "reduction" until I joined the FAOL BB and started reading about furling and how others approach it. I only mentioned it in the above post because it seems to be part of the language which others here understand and expect.
As to "how do you determine how much length to add to the layout to allow for reduction?" I don't. That is more scientific than I am interested in, the way I see it, because you are trying to establish a specific finished length, and I am not. I am not at all opposed to whatever science others choose to apply to the furling process, but I am also not governed by it. I can make leaders from a few materials that I like that perform well for me, and that is my goal and I meet it consistently.