If you are talking about the Sympto-thermal method of NFP it's not as cut and dry as that. It's based on your earliest ovulation of the last 12 cycles, plus normal luteal phase length, plus mucus observations and temperature. They all have to be combined to have an accurate window of the fertile period. It varies for every individual woman. That's why the method is effective.
For example, my shortest cycle is 26 days, but I always ovulate later than day 14 and have a 12 day luteal phase. So my fertile phase is from CD 6 until the 3rd day of temp rise. But I can also go to CD 8 or 9 if I do not have any mucus because I don't usually ovulate until later on.
Me, NFP user 8 years.