I know that the female cycle is 28 days or 30 days long and that ovulation (or release of the egg) happens in the middle of the cycle. The cycle days are counted from the first day of the period to the 1 day of the next period. Also, the egg can live up to 2 days only and the sperm can live up to around 7 days. Therefore, following NFP, we usually abstain from sex from the 8th day to the 21st day of every cycle.
My question is that since my wife’s cycle and period start date is differing every month, can we use the same calculation every month. For example in February (2020) her period started on 19 Feb, whereas in March (2020) on 15th March, earlier in December (2019) her period started on 26 Dec. and in November on 29 Nov. (2019).
In this case, can we still apply the rule that we abstain from sex from the 8th day to the 21st day of the cycle to safely avoid conception?