Question from 1st tri: Figuring out due date/how far along

WifeofaMarine

I asked this on first tri, but I wanted to get your input too. When calculating your due date/how far along you were (before your first u/s), did you count the first day of your LMP as the day spotting started or the day full AF started? With my LMP, I started spotting Jan. 16, but full AF didn't start until Jan. 20. I was charting, and according to FF (using temps and CBEFM readings), my O day was Feb. 3 (CD 15). I won't have my first u/s for awhile, and I know they are only a few days apart, but just curious which is probably more accurate to use.