So when do you actually ovulate?

Shekiba • 3 yrs TTC #1. MFI. 1st IVF cycle in July '17 = 2 embryos by ICSI frozen. Natural FET in October canceled. Medicated FET with both embryos in September=failed IVF cycle.
So I see everyone saying _ days post ovulation(DPO) I got a +opk on 6/8. I always take on the day after to make sure. So on 6/9 it was -opk. So my question is how do you know when you actually O'd? I know it "says" 24-48 hours after a +opk but how do you pinpoint your calculation?