The first day of blood flow, not spotting, is day one. Understanding Your Fertility is a GREAT book, highly recommended.
Yes, I would count that as your first day. We are most fertile from day 12 to day 16 so even if you're off by a day, you'd still have a pretty good chance, I think. Ever tried ovulation predictors?
I always wondered exactly how you count that. I sometimes spot a little first, then it starts flowing usually within a few hours. So for me I have always counted that spotting as my period because it shows up fairly quickly.Maybe someone else will have an exact answer. =)