When should I test? Struggling here!

My last cycle started on September 23rd and next AF is predicted to start November 4. I'm irregular but I usually have af every month just never really sure when. It ranges from 29-39 days. So it's hard to determine when I ovulate and when I should  test. I used an opk and every day was no line and one day a very faint line (7days total) so that told me nothing other than I didn't ovulate during that week.  Help/advice please.