Sweet Cherie- they say that a true positive OPK is when the test line is just as dark or darker than the control line. However, I know A LOT of woman who never actually get a 'true' positive, theirs get darker but never as dark as the control line. My first cycle using them my test line got pretty dark, but never as dark as the control line. I still ovulated, and AF showed on time. This cycle my test line has gotten just as dark as the control line, I was just surprised because I wasn't expecting it too.
Not sure why one cycle it gets darker than another cycle... hoping it's a sign of an extra strong eggy!!!! Lol.
I start using OPKs around CD7 because I like to see the progression of the test line, probably the best way to get to know your own is to do the same- start testing early and watch for when they get darker and are detecting your surge!! Some woman have more than one surge though, so don't forgo BD'ing until you know for sure you O'd
After your first cycle it gets much less confusing and you aren't second guessing yourself because you have something to compare it to!