Hmmm....you may get more informed answers in the TTC section.
I have a hard time pinning down my O, so I start using OPKs 14 days after the first day of AF until I get a positive. I've only been doing this for two cycles though. If you have a general idea of when it is, try testing twice in a day because sometimes you miss the LH surge by just testing once.
Good luck!
