kk04,
It really depends on your cycle. If your cycle is 28 days, your ovulation will be taken place on the 14th day after your 1st day of M. Then start to do it on the 12th day after 1st of M... I would suggest you to do the ovulation test to see when is the best time to do do. You may buy the ovulation test kit at Watsons and other drug stores.