You want to know when you should stop testing, you need measures of expected defects count, to know if you already discovered all defects or still more testing is needed. It can help you also to measure the product quality at any certain point.

The answer is in a book named, Software Project Survival Guide by Steve McConnell, chapter 15. Fortunately, he has an online article that explains different techniques in plain and simple language. Look at: Gauging Software Readiness With Defect Tracking [http://www.stevemcconnell.com/ieeesoftware/bp09.htm]