Thanks for your detailed response. Your process does make sense to me. Also, someone else clearly ascertained the % reduction through trial and error, and left behind the requisite peg spacings, etc., necessary to make a functional leader.
Yes, I do try to make uniform, predetermined length leaders, and am quite satisfied with what I produce, as they are all uniform. Skip Shorb was my mentor, and he used(s) a 10% reduction factor. As this results in a functional leader, but one with a 'loose' furl, to my thinking, I went in search of methodology for getting a tighter furl, similar to what rope (lariat) makers get (a process that I was introduced to during my formative years in the '40's). This search led me to a paper that addressed maximum twist in ropes, wherein it was noted, that based on physics, the maximum reduction for a 2-strand rope is 37%, and for a 3-strand rope, it is 32%. Based on this information, I proceeded experimenting with various % reductions beyond 10%; all the way to 33% for a 2-strand rope (during this time, I did not try to make a tapered leader, as I was primarily concerned with the 'stiffness' associated with each level of reduction---I have seen new lariats that could stand on their own on their ends they are so stiff!). After numerous trials, I finally settled on a 20-22% reduction as optimal for a supple leader. Anything much beyond about 25% is way too stiff, IMHO. For my own leaders, I use a 20% reduction; which is extremely close to your estimated 18% reduction.
I too use the materials that I personally prefer. As a consequence, I use only polyester threads (Guetermann). And as far as how many furlongs one can run in a fortnight, I have no interest. I make them at my leisure!