I think there is no way of telling - I'd just BD quite a bit to cover all bases
It's probably safer to assume CD 13 for BDing purposes, and CD 14 for deciding how early you can POAS for a pregnancy test
The month I got pregnant I Oed earlier than the previous few months, and therefore got a positive the first test I did in the month (and we hadn't started BDing yet...) so I basically jumped hubby then and there
and then the next morning, the morning after and the evening after, by which stage even at the latest possible O time it'd be too late to keep on BDing.
Good luck and have fun