Determining DPO!

Hi everyone, so I have a question which may be a dumb question but I want to have a clearer understanding! I am using the Clear Blue advanced ovulation tests, and once you get the "peak fertility" smiley face, when do you start counting DPO? Cuz once you get that face, you may ovulate in 12-36 hours? I am also tracking cervical mucus but nothing else. Any ideas? Thanks for your advice!