I have to agree, this is not an acurate type of birth control at all, each cycle can be different as well and not everyone will ovulate on day 14, I can ovulate anywhere between day 12-16 and I have a regular cycle of 29 days, your fertile most just before you ovulate as its better to have the sperm waiting for the eggs release, you would be much better off using condoms, you will no doubr get pregnant trying to use this method alone as a form of contraception
Because sperm can live anywhere from 5-7 days I'm females body so it covers any of those days and to be honest that is the worst tipe of birth control I've heard of you should consider a different method