From what I've read you count cd 1 as first day of heavy red flow.

Good luck getting your girl I got mine but wasn't fully committed to the method but I can share our dates as I was tracking O with opks as i wanted to make sure we were dtd at the right times for optimum chances of a girl. just keep in mind my cycles were all over the joint which is why I don't o till later in my cycle.

Cd15- neg opks. DTD.
Cd16 - faint pos on opks. DTD.
Cd17- darker positive opk
Cd18 - def positive opk. DTD.

Did not DTD again until cd23. So as soon as I got that positive, we DTD and then made sure we didn't DTD again for a few days, hoping the girl sperm would have a chance lol worked, I'm 25 weeks preg with a feisty little girl

Good luck!