My wife and I have been there... Our problem using a calendar was that my wife's cycle is long and abnormal (40-45 days) If you want to save yourself some time and energy... go to this site and order ovulation test strips... your wife pees in a cup and it tells her if she is ovulating today or within the next 24 hours. It worked for us the first time.
http://www.early-pregnancy-tests.com/combopack.html