you measure the length of your cycle by counting from the first day you start your period, so the 27 oct is day one, then it finishes on the day before you start your next
period, so if your next
period started on the 25 nov, you would have a 29 day
cycle, you would generally ovulate anywhere from day 12-18 of that cycle, you work that out by also counting the days from when you started your period, also keep an eye out for fertile cervical mucous, overay cramps etc, that would give you an indic