I used a tut from hair-hardware to learn. I now sew instead of glue though. I have measurements written down now for the different sized bows I make, but, to get to that point, I just eyeballed it to see what worked. When I found I had the right size I wanted, I just put it down on paper so I could remember.
ETA link: http://www.hair-hardware.com/articles.asp?id=133