Do you count dpo first or last day of ovulation!??

I have been counting my dpo as the last day of ovulation. Since I try to have sex on my most fertile days.

So right now, from the last day of ovulation, I would be 12 dpo.

But, if u counted from the first day, I would be 18 dpo.

But I didn't have sex for the first few days of ovulation.

How do I count my dpo??