I would tentatively say you ovulated 12-24 hours after the darkest line and you had a long surge. I always had very short surges and would sometimes miss it altogether if I tested at the wrong time. That said, I don't believe OPKs are the be all and end all of ovulation monitoring. Your body can tell you more than a cheap test, the tests are useful to help pinpoint it among other signs, such as egg white or super clear runny cervical fluid, basal body temperature etc. Get to know your body rather than putting all the emphasis on an opk.