Not sure how far along I am

My cycles are really long, 45 days typically. My last period started August 9th, but I wasn't set to ovulate until the beginning of September. I took an opk, and believe that I ovulated either September 6th or 7th. I got my BFP on September 16th. If I go by my last period, that would make me 8 weeks tomorrow, but is that incorrect because of my long cycles? It would be 4 weeks tomorrow since ovulation, so that's what I keep going back to in my head. We have our first ultrasound on October 13th to help date the pregnancy. Thought I would see if anyone had any thoughts/suggestions on how far along I would actually be!