OPK when/how to use

Hi ladies. So I'm slightly confused. I have a pretty regular cycle of 45,46,47 days. Which is a long cycle to be seeing when I'm ovulating. My OB said no matter the length of cycle you should ovulate between day 10-18 and use the opk during that time. Then I read you usually ovulate two weeks before your period is due. Which is correct? Im not trying to spend a fortune testing at the wrong time. Also what is the best way to use them? I read it says not to urinate four hours before using them. Any input is greatly appreciated! Baby dust to all!