You don't need to get too hung up on what "cycle day" it is.
You just need to take it for the minimum number of days known to provide endometrial protection.
If you are still having a menstrual cycle yourself, it is generally suggested to time the progestin to coincide with your luteal phase so that the withdrawal bleed overlaps with menstruation.
However this can be very hit and miss because the 28 day menstrual cycle is rare outside of textbooks and more so in perimenopause.
Therefore if you are reasonably confident when your next period is due, you could start the progestin two weeks before this.
If you are no longer having regular periods, or find yourself losing track, simply using day 1-14 of the calendar month can work out better.
Where people often fall down with sequential HRT is getting so preoccupied with waiting for day whatever to start the progestin that they end up undertreated as weeks have gone by without reaching that day, or they stop it prematurely if they begin to bleed during the course.