Yes I'm aware that's its not a 100% but It's worked for me! I didn't use OPKS to track it, because i'm not trying to conceive. But if I was it wouldn't matter either way. Just not trying ya know :)
I usually ovulate around the 12th and 13th
The pullout method works 100% of the time until it fails. I don't really care which method you use either way, but the only 100% reliable form of birth control is complete abstinence.
But you're getting BFN's, so you just O'de late if you've O'de at all. It happens. And since you've already said you're cycles can on occasion be 5-6 days late, then you just aren't as regular a you think you are.
Test once a week until you hit 60 days, and if you're still getting BFN's call your doctor to just start your cycle.