How to calculate

How do I figure out my DPO? I switched my <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> to pregnant and it took my fertile days off the map! My last first day of my period was the first. Got my BFP last Sunday . I am just trying to figure out more about when I could have conceived. TIA🤍