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I'd really like to learn the proper numbers on this. What are the precise risks of a timing hiccup or second ovulation during a menstrual cycle? Also, how does regularity of one person's period affect these risks? How accurate are the indicators for ovulation most methods use (body temp, cervical mucus, etc.)?

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Well... it depends.

If you only rely on this method for a few days each month immediately after the end of menses, the chances of it failing are MUCH lower than if you rely on it for a week prior and a week after menses.

That said, wiki claims[1] that the latter approach is >95% effective in perfect use, and 88% effective in typical use.

Compare this to withdrawal[2], which wiki claims is 96%/78% effective respectively.

[1]: https://en.wikipedia.org/wiki/Calendar-based_contraceptive_m...

[2]: https://en.wikipedia.org/wiki/Coitus_interruptus

You linked to the rhythm method, not fertility awareness. Fertility awareness means actually measuring the physiological changes resulting from the hormonal changes that precede ovulation (temperature, saliva, cervical mucous). This is much more reliable than guessing based on the date, which is obviously going to fail relatively often as ovulation isn't reliably regular even in the most "regular" cycles.

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