Which natural family planning method is based on Ogino Knauss theory ?

In Knaus-Ogino method, the feile period is determined by subtracting 18 days from the shoest cycle and 10 days from the longest cycle which gives the first and the last day of feile period, respectively. This method will result in approximately 25 pregnancies per 100 woman years. The failure results from irregular ovulation or from irregular menstrual cycles. Some couples prefer this method on religious grounds or because they find other methods unacceptable. Reference: Shaw's Textbook of Gynaecology, 16th edition,page 265.