Hi Sassy
I think the dark line means that you are just about to ovulate (between 12 and 36 hours)
It is best if you have been b'ding before that as well.
I used to ovulate around day 16, I would start OPk's around day 13 where i would get a faint line, and the line would get darker every time i did it, therefore I would know when to bd!
Hope that helps