They aren't going to count from the date of your miscarriage...that isn't a period, and short of using ovulation tests, there isn't a good way to tell exactly when you conceived.
In a "normal" situation, you aren't actually pregnant the first 10-14 days of your pregnancy. They count from the day of your last period because that is a KNOWN day--women's cycles are all different and we all ovulate at a different point.
You've only had to have conceived two weeks or so to get a positive pregnancy test...the day of your missed period is only 10-14 days after ovulation and conception (assuming a 28 day cycle).
They are going to take HCG levels, the date of your positive pregnancy test, and the date of your miscarriage to calculate an approximate due date--add in ultrsound results later. You'll be able to tell more as you get a little further along.
Given that normal human gestation is 38-42 weeks...you've got some time to figure it out and it will still probably be wrong.