I don't think you are being obsessive, just meticulous. I wish I would have been more discerning with the charting process a long time ago.
I actually wondered the same thing today when checking CM, since we BD'd this morning, and I am still waiting for a + OPK. Here is a recent post I found regarding the issue, and it seems the consensus is it requires 12 hours to know for sure:
(sorry not clicky)
Also, there was a bit on fertility friend I found, not sure if it is helpful. I have to imagine some others here have dealt with the same.