You need to count the days from ovulation to the first day of full flow period.
That means you need to know exactly when you ovulate. The best way of doing this is to take your temperature everyday with a basal themometer and put the info on a graph (charting) If you are interesting in doing this have a look at www.fertilityfriend.com to learn how.
There are other "clues" to ovulation but these are not absolutely accurate:
-some people feel ovulation pain in their ovaries
-cervical fluid (EGCM) changes to become stretchy and slippery and more abundant
-the position and texture of your cervix changes, if you feel it (you have to be brave for that one 
) you can notice the difference
-you can use ovulation predictor kits (these will only tell you when you are likely to ovulate, not that you definitely have ovulated)
If you want to know more you can read all about it in the book "Taking Charge of Your Fertility" by Toni Wecshler.
best wishes xxx