This method is based on an observation that the ovulation in females takes place 2 weeks
before menses. Now as we know that the menses cycles in women may not be on precise
day every month, the calculation of the safe period is done on the basis of duration of 12
previous cycles. First note down the shortest and the longest cycles during the last 12 cycles.
The first fertile day will be minus 18 days from shortest cycle.
The last fertile day would be minus 11 days from the longest cycle.
You can see examples for calculations of your safe days in related links (see below)