I think everyone is entitled to have an opinion, it's not guess work at all, it's very predictable.
Yeah you see no its not predictable at all
For the minority then yes, their cycles will be like clock work. For the vast majority, however, and for a million different reasons, there will be fluctuations.
Then there's flow. Periods can be predicable for months and and boom, a canal gate opens.
If it were an exact science then so would conception.
Humans are annoyingly difficult to predict at times.