First and last day of periods,how do you know when to mark on your calendar when they started and ended?

K
My periods start off really light with brown spotting do I mark that as my first day of period? Or does only actual red flowing blood count? And same for the end of period, is the last day when there's no more spotting at all or the last day when there's no more flow?
​I just want to make sure I get it right so all the timing for ovulation prediction is close and accurate.