DPO questions

How many days past ovulation should I take a PT? I tested on the 16th and it showed ovulation. Made love a lot before and after. So days post ovulation would mean days AFTER the 16th? Or days after ovulation cycle? I'm aware that regardless if I would have taken an ovulation test after the 16th it would have been positive as well and mybe would have been positive a day or so before? Once it's positive, how do you know exactly how many ovulation days you have?