The original 220-age=max hr comes from a cardiologist who did a study on his patients in the early seventies. It is really only a ballpark figure, as all of his patients had cardiac issues of one type or another.
Your HR monitor should have a fitness test that allows you to figure out your personal max HR based on your HR during exercise. That may be more accurate. Like anything else, as you gain experience with HR training, you will figure out what works for you.