This is a kind of guessing game, but for most women it actually 14 days before your period.  So looking back count 14 from the start of your period back and it should be about the same time.  If you happen to be lucky and get your period every 4 weeks (28 days) then it would be 2 weeks from the start of your period counting foward as well.  But it's really based on about 2 weeks from ovulation to when your period starts, so going back the 2 weeks will give you a better idea of when you will ovulate.  Also, when you count, take note of the day, and when that day comes, see if you are a bit wetter and it's more egg white in consitancy.  This is when you are most fertile, and it should coincide with the days that you counted out.  Good luck and hope this helped you!