Last month, I had a 20mm folicale on the Thursday at my Scan. Got a positive OPK on the following Tuesday, but I think that I was probably about 24 hours too late & I just caught the tailend of the surge. I did get a questionable opk on the Monday so it was one of those things. My period came exactly 14 days following that, which does explain that I'd probably missed ovulation by at least 24 hours, maybe even a little longer.
So, with that wisdom. I think that I ovulate when my folicals are 22mm going with the basis that they grow a mm a day. Ovulation is to be expected between Tuesday and Thursday of next week.
I'll keep track with the opks, not sure how I'm going to manage that in work but we'll see!!