It hard to grasp because it is stupid.
It is laughable seeing you own your mansplaining and think you are correct.
Women tracking their periods from the end of one to the beginning of the next IS THE BEST AND EASIEST WAY for them to identify a pregnancy risk. it is time honoured and a good way to do it.
You saying there's a more technical way to do it, because technically the period is not the exact date of fertility, when the woman would not be able to KNOW or TRACK what you are indicating normally is just the height of stupid mansplanning.
Anyway now Manspain how a women would go about tracking it the way you say instead women should track it?