Due date question

Before taking BC for many years I had irregular periods, and my cycles were usually 40 or so days long. I stopped BC mid March and since then I have only had two periods which were 39 days apart, the last period being June 7th. On June 22 I spotted and then I had a negative pregnancy test on July 4th and July 17th. I figured my next period was due around July 15th. I had a positive test on July 24 and July 25. I have no idea when I ovulated, because if I did ovulate on June 22 then I probably would have had a positive test on July 17th since it was so many dpo. I did a due date calculator by my period and it said March 19th. I head once you miss your period you are already four weeks pregnant, but what if your cycles are longer like mine? I don't see the dr until August 19th, so I am just trying to get a better idea of what my due date will be. Thanks!!