It varies for everyone and sometimes cycle to cycle. I usually ovulated CD11 or CD12, but it varied between CD9-CD14. Try using OPKs from soon after af finishes, once they turn positive you should ovulate within a couple of days of the positive. But that varies too, some ovulate the same day, some two or even three days after and sometimes you have more than one day of positive OPKs. Only real way to know when you ovulate is by temping each cycle to get an idea of your usual pattern