It all depends on what type of calendar you are talking about. If it is the standard 28 day paper calendar which marks the first 5 or 7 days as your period and day 15 as your ovulation date, it is not accurate unless you have a very average cycle.

If you are talking about a chart where you track your basal body temperature, cervical mucus and cervical observations, they are much more accurate. These types of charts are normally kept on a computer/web and it will guide you and help you to determine ovulation and your fertile and infertile times. The more cycles you have in it, the more useful and accurate it becomes.

Your other option is just to use ovulation prediction kits (OPKs). That way you can see how accurate a normal standard and average chart is for you.

The consensus today is that you, manton9878, will always ovulate the same number of days before your next period. This differs for every woman but is LP is constant for you. If you have irregular cycles, it is the first part of your cycle that differs in length (follicular phase - period to ovulation). Unfortunately you can not predict the future to know when your next period will be and subtract your LP length from it to know when you ovulate. So all you have is your ovulation indicators (mucus and BBT) and your past history.

Best of luck!!