What is the ruling when the habitual number of days is half or less than half of the assumed period length?

Primary text

If the habitual period is half the time or less, there is no definite menstruation. This is because if she has five days of bleeding, it is possible those five days were the first five, or the latter five, or a combination. Therefore, she observes five days either through estimation or from the beginning of the known time frame, according to differing scholarly views.