Nobody wants to miss their project deadlines, so a common practice of project managers is to pad their project schedules by an arbitrary margin, like 10 percent. This works in a pinch, but there are better options available to you. In a post at the Project Risk Coach, Harry Hall discusses the value of applying quantitative risk analysis to the project schedule. He shows how using some simple calculations and charts can create a more precise and reasonable margin of buffer days, one backed by demonstrable data.
You can view Hall’s concise explanation here: http://projectriskcoach.com/evaluating-project-schedules-utilizing-quantitative-risk-analysis/