Can anyone help me calculate my due date please?

Disclaimer: I am calling my doctor Monday morning since they are closed, but wanted to see if anyone had the knowledge/experience to help me estimate this before then.

I had a baby 3 months ago and have been exclusively breastfeeding my him. I have not had a period back yet and had no idea I ovulated or when. Could anyone help me estimate a due date or how far along I may be?

Conception date would of been Feb 29th.

The first very very faint line I got on a test was March 17. Today I had a stronger line.

Would I be due around dec 10th ish? Or what would your estimate be? Any thoughts welcome thank you so much