Just a question...

Deidra Lanae • 20. |Cancer (July 3)| Due December 2 💙

Does anybody know kinda how to calculate a close to accurate conception date? If my due date is December 2nd. I've heard some people say March 2 or 3. But on this app it tells me March 11th. ?? While, another app I have tells me March 14th. I know I won't know EXACTLY but what do you think is closer to true?