Hi Sadie,
I'm not sure if I can answer your question, but if your hubby has a normal healthy sperm count, sperm can live up to 5 days after intercourse. So if you BD even 3 or 4 or 5 days before ovulation, you can still get pg. I didn't use the OPK that show up a line, I used the ones that simply show a happy face just before you're going to ovulate, so they are very accurate and you can't miss O time (they are expensive unfortunately). GL to you!
B.