I didn't have a LMP before this pregnancy, so we couldn't calculate due date from that. I just sort of guessed, and I figure better to guess less rather than more (so I was actually guessing 5 weeks when it turned out I was closer to 8). If it helps at all, you can consider the time that the test works in (most say on the box, like "works within days of your first missed period (2 weeks pregnant)" because it sounds like you are right on the edge of the test accuracy. For me, I didn't actually have a firm due date until they did ultrasound and gave me one. Up until then, I just went by month. "When are you due?" "Sometime in August."