Just wanted to know (See above)

Calculate from the start day. This guide might be helpful: How to Determine First Day of Menstrual Cycle: 3 Steps (http://www.wikihow.com/Determine-First-Day-of-Menstrual-Cycle)

Keep a calender in your closet and mark the days when you have your period... that way you have an easy record of when and how long... and you won't be relying on memory.