Last period or conception date


I’m confused at which I should be going by to get my most accurate due date. When I use my last period, it puts me at 6 weeks 1 day but I know I can’t be that far along. I track my ovulation and got a positive OPK on July 28th. I’m thinking I’m only around 3weeks and 3/4 days if I go based off conception. I have super irregular cycles, typically around 50 days. If someone could maybe try and help me understand this, it would be greatly appreciated! 🤦🏼‍♀️