1/2" too long is way worse than 1/2" too short, by leaps and bounds. That being said, doing the math is one thing. That gets you close; some place to start. Now, do you use a string loop? If so this stretches you out that much more. A release that is not adjusted short enough stretches you out even more. And how do you know your bow is really set for a 30" draw? Have you actually measured it or are you just assuming the factory has it right? Don't count on it--most draw long.

Do you get what I'm saying? It's not just about the draw length of the bow. There are a lot of things that help determine how and where you anchor. The whole idea is that when you have everything adjusted closely your body geometry allows for better bone alignment and less muscle tension to draw, aim and shoot the bow.

Now I'm probably going to get into trouble but my suggestion would be to log in at www.archerytalk.com and do a search on "shooting form" and pay real close attention to posts by Nut&Bolts. Maybe a search of "Nut&Bolts" would work, too. This guy is about the best GURU that I know of when it come to explaining these things; complete with pictures and such.
I know he's way better at it than I.