Which of the following statement about calendar method is false ?

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. Not related to cost Ref: Shaw Gynecology 17 e pg 253.