I had a string jig I bought at a trad shoot in the '90's that worked great. When I got back into the sport last year, I couldn't find it so I downloaded a plan for building my own from the web. After I built it, I had the same problem as you when I first started. If your string is coming out too short, measure and adapt which nail you use to allow sufficient string to compensate. It worked for me. I built a few before I got it just right, but now it works like a charm!