I’m a little confused on how to calculate my “DPO”

So according to my app I have one day a month that is my actual “ovulation” or “peak day” but if I have my “fertile window” where would I start my “DPO”? Would it be from the beginning of my window or my Ovulation day according to my app calendar?

The way I test my ovulation is with the urine sticks, so I technically don’t know my “peak day” which kind of confuses me on how many days I’m actually post ovulation.

I hope this makes sense enough for someone to help me out on understanding.🤞🏼