Oh yeah, I totally understand! Then I would recommend from my own experience that you test more often than once a day. You can get cheap OPK's on Amazon (I use these) so you do have to worry about how many you go through in a day. My approach has been to test 2x a day starting around CD12 but if you know you ovulate earlier maybe start testing a day or 2 earlier. In this last cycle I got a positive OPK at 11pm and another at 8am. I must have surged overnight. If I had tested only in the afternoon I would have missed the surge altogether!

Good luck testing!