Ovulation is 12-16 days PRIOR (an average of 14) to your period
, which means you cannot guarantee the calculation
forward (only looking backward) unless your cycle is always the exact same. I.e. If you start af on 11/30, you would have ovulated between 11/14 and 11/18.
The best way to calculate is to check your basal temperature (BBT) and use an ovulation prediction kit (OPK). The ovulation prediction kit will advise 24-48 hours BEFORE you ovulate, and the BBT will advise AFTER ovulation.