For ladies who have had IUI

Rayna • 38 years old and been blessed with two little boys, one through IVF and one conceived naturally, both with countless prayers.
I am wondering how you calculate where you are in DPO? What do you consider your ovulation date?  36 hours after a trigger shot? Date of the IUI?  Day of positive OPK?  Day before BBT rise? Sorry, I am just a bit confused.