For me.... it's just a guess and it works just fine. I leveled the fuse on a stand with just my eyes and put in what I thought was about 4%, using just my eyes. I don't think it is a critical measurement and you can always change it after the first flight. Just my opinion. I got about 9 great flights on mine.
http://www.youtube.com/watch?v=WilrzKCXNhk