Aclient states that she wishes to use the calendar method of birth control. the nurse concludes that the client understands how to calculate the beginning of the fertile period when she states, "i will:

The correct answer is "I will subtract 18 days from the length of my shortest cycle."
If you want to calculate when your fertile period will begin, the best way to do that is just to subtract 18 days from the length of your shortest cycle. If you subtract 11 days from your longest cycle, you will get the last day, and not the first one.