When you ovulate very much depends on the length of your cycle. For example, if your period arrives every 24 days instead of 28, you will ovulate sooner than someone with a cycle of say 32 days who may not ovulate until day 20 so it is very much an individual thing.
Do get that book reccomended by the other lady, it really is worth the money.
It is annoying that they use the 14 day thing, I knew the exact date of conception on a pregnancy last year but they seemed obsessed by the date of my last period. The scan was spot on with my date and way of on theirs. Sadly, we lost the baby at 13 weeks and have just lost another at 10. We hope to try again with the knowledge we gained from the book. All the best.