I agree with the others - cd1 should be the first full day of red flow (full day meaning you wake up to it or it starts before 12pm).

If it's any help to you, I have pre AF spotting (due to low prog levels) for up to 7 days some cycles before arrives "properly" and I have always counted cd1 as the day of full red flow, I don't count the spotting at all - BUT this cycle I think my gyno worked out cd1 on the wrong day (2 days earlier than full flow) but it didn't affect ovulation or anything like that (my chart says o occured on cd16 rather than cd14) so at the end of the day if your cycles are regular and you are pinpointing when you o it shouldn't make a difference.