+1 on DC's post.
Not sure what kind of jig you are using, but you can adjust the length of string that you wrap over for the loop and dial it in that way. So if measuring 8" from the end and your string is coming out too long, try 8.25" on your next one. Like has been mentioned, there's some trial and error, but you should be able to get real close after making a few. Keep notes on what you try.
And those ones that don't come out right, you can keep them for tillering strings. I have about 10 or so at various lengths to use for tillering.