How can one tell when the egg was fertilized?? How do you really know DPO

If there is a week that you could be fertile, and within that projected week there's only 24 hours that the egg can be fertilized... How can one tell when to start counting?? I have used this app to track my cycle for many years. I have had spotting 3-4 days before the projected ovulation date. My cycle usually starts either the day before or the day of expected date. I'm very regular. This cycle we BD day 2 of my fertile week and the day before projected ovulation date.

I don't track ovulation with temp or ovulation test. I did that for two years and never got pregnant. I stopped and got pregnant. I think it was too much stress.. but now, I don't know how to track dpo...

Facts I know...

#1 sperm can live for 5 days after ejaculation

#2 it takes approximately 5 days after egg has been fertilized before implantation

So.... When I look at my calendar I don't know when to start testing... Can anyone help? 🤦🏾‍♀️ I feel so clueless...