After you chart for a couple months you will start to see a pattern and have a really good idea of when you are going to O.
Honestly though... just BD every other day and you will have all bases covered

We aimed for the every other day method last month from around CD 10-20 and got a BFP. (Short lived... but still a BFP!)
I think I was stopping the BD too soon on my previous cycles. We wore ourselves out trying to get as much baby making in before O as possible that I think I was missing my actual O date. I am pretty sure I O two days after FF says.
Okay I think I got long winded.... LMAO.
OPK's are great and I REALLY suggest you try them..... when you are TTC *wink* of course