Hi Sally,
I am never able to get a true positve opk.
I get a darker 2nd line but never quite as dark as the 1st line. A friend who just got a positive HPT the other day is in the same boat, she only got a faint 2nd line mid cycle and voula! Dont let it stress you out and worry about weather or not you are ovulating. The first month I thought that I may not be ovulating but it wasnt the case. I know that I do ovulate but opk's are just a bit slack at finding the surge.

I usually test once a day 2pm after not drinking any H2o for at least two hours from cd10 then as I get closer to midcycle and have other signs ie, EWMC I start to test at least 2-3 a day, FMU, 2pm and 8pm. I think you might find that if you get some affordable opks (ie lullaby) test a lot mid cycle even using first morning urine then it will help you to know your pattern. Then you'll feel more confident in following months (if you need them that is) you can know when you are likely to get a positive and use fewer opks to get it.
I go by the darkest day.
I know the manufacturers say the 2nd line must be darker than the control but I am not alone in believing that they might be wrong.

Hope that helps, sassba