Oh Tictac,

If only we could live in a perfect world where we all O'd on day 14 and had 28 day cycle, how easy it would be.

I don't think there is a precise answer to this. I'd be inclined to take it as the shortest length cycle, because then the calendar will predict the earliest possible day and you can take it to be also within a range of a few days after that as well. A cycle length which varies up to 5 days is perfectly normal.

Do you have other signs of O? I'd also be inclined to get acquainted with physical signs of O so you can pinpoint it more accurately.

GOOD LUCK.