Due date calc for long cycles

AJ

Hi ladies. I have my first prenatal appt and ultrasound on October 9. In the meantime, when trying to estimate my due date, how should I do this? I have a longer than average cycle (33-34 days) with a shorter LP. I didn’t ovulate until CD 23. I just feel like going by the first day of my last period might make my estimated due date too soon...

Thoughts?