How to Calculate Due Date

Alexis

Probably a dumb question… I feel like I have a handful of different dates based on these app date calculators. Just got bfp last night. AF started Dec 5th. But this is a different due date then if I put in how long “usual cycles” are. But my FF app says I ovulated earlier then what I think this app uses. Do you use O day as conception date? Is that most accurate?