I twist in a loop, the smaller one, and put it in the knock, then twist away on autopilot while watching discovery or something. when i get to the other end i lay my hand across the belly with one side of my palm on the knock. I twist in the second loop, the bigger one, and make the end of it touch the inside edge of my hand. not scientific, but its worked so far. i rarely have to twist more than a few times to adjust the length (except for after the first good stretch, cause i don't use prestressed string). most of the time i just retwist the second loop and inch shorter after the string stretches, cause i don't like twisting the string that much, as it weakens it.
its no amo...but its worked everytime for me