Newbie BBT question

I just started tracking my BBT. I have PCOS and irregular cycles. This is my 3rd round of clomid @ 150mg this month. My PCOS makes regular OPKs look nearly positive all month.

What I'm confused about is when tracking BBT and looking for the "3 higher temps" doesnt that mean you wont know you are in your fertile window until after you ovulate? How do you not miss your fertile window/ovulation?