And, if you have a program like Promash, or do a little simple math, which I do not recall right now (I'm sure someone can help me out here) You can take a preboil gravity, and volume, and see just how long you need to boil to reach your target gravity. or, if your efficency was extra high that brew day, add some water to thin it out. Do this, and you can hit your numbers spot on, every time.