How do you know DPO?

So I was using Clearblue advanced ovulation. I had 8 days of flashing smileys before I ran out of tests and never bought more because I was frustrated and they’re expensive (we BD the 4 days leading up for glow predicted ovulation and again yesterday (after 3 days off) in case I ovulated late).

I think I missed up the reader by testing too often and not doing long enough holds (this is the only opk that says specifically to use FMU). When I ejected the strip and let Glow read it said positive the day of my predicted ovulation in the app, and I had EWCM and high sex drive. But I never got a positive test...

Thoughts? How would you determine your DPO?

Do you think:

1. Just didn’t ovulate this month

2. Didn’t continue testing long enough and probably ovulated a little later

3. Should go off of Glow’s predicted ovulation

4. Other- Tell me in the comments!