I have been making my own FL and fishing them for over 10 years now. My board is auto with a geared motor set up.
Not wanting to be confused with percentages, etc, I feel a FL should be of a tightness of just before it breaks. So, I rope up my thread on my pegs for each material I use. run the motor with a stop watch running, until it breaks. Look at the time it took until breakage, then subtract 5 seconds. This is the time I will twist this material for my FL. Yes, I scrap one mess of thread for each different material. I have found that different materials get to the breaking point at different times. For me this is just a simple method for control. I do not care how many turns or what weight it needs or what percentage it shrinks.
I only make and fish two different FL. I use 6/0 uni for my dry fly fishing with a wt fwd line and 3 to 5 ft of tippet. I do grease the FL w/red mucilin. (only to keep it from sprinkling droplets of water when casting to a rise.)
When fishing my lakes with a Nymph, I use a Rio Lake line Intermediate, that sinks at a specific rate. Nothing I did would sink at the same rate until I used Berkley Trilene 100% Fluorocarbon, 2 lb which is .006" dia. It is a bit pricey but does sink at the same rate as my line and I find it works quite well for me. (Note: I have never found Vantage in 2 lb. I tried Vantage 4 lb and do not like it.
My first experience was watching Hans flurl a leader by pinning (safety pin) thread to the motel room carpet. Twisted with a battery powered drill motor.
There are many different ways to make your own. Nothing is graven in stone. I suggest being inventive like many have posted. Enjoy the making of your own FL. I will never go back to any other leaders. Now I know some prefer other leaders. So be it. Just go fishin and have fun.