I just made a string jig from some internet plans. The measurements are off about 2" if I follow the formula that came with it, so I've made adjustments by making more strings and keeping good notes on what I did. Usually I will get it on the second try, but sometimes it takes 3 strings before I get one the length I would consider acceptable.
Once I got a few strings made that turned out the right length, I used the notes I kept when making them to mark my string jig accordingly. Now I use a flexible tape measure (the kind tailors use) to measure nock to nock, and they come out right. The good news is, while I got my jig adjusted accordingly, I really learned how to make a decent string!
Ron