I was very irregular before the pill, that's one of the reasons I got given it, plus PCOS.
I didn't take it, and counted days to have a rough idea. And it came within what would have been the 7 days wait from the tablet.
After than I entered my first period day on this iPhone app called Maybe Baby and and the length of the cycle since the last period, and it calculates what your next expected period is, what your expected ovulation day is and what your fertile days are.
So far it's been spot on with the periods;not so sure with ovulation as I haven't really been charting temperatures etc, I find it a bit too much....