You O an average of 24-36 hours after your FIRST positive, not the darkest. The surge is a message, and your body "hears" it at a whisper even if it starts shouting later on. :)
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When in doubt, BD. This is because you can conceive from an earlier-than-pos-opk attempt, but NOT from an after-O attempt. I hate to see you guys waiting on darker OPK because sometimes they just never arrive, so better to be in with an attempt day before positive OPK (O-2, also great odds conception) than waiting and missing it.
Girliecat, you were covered either way (that's the point of hte e4d method),, this is just for general informational purposes.