I agree, it is defnitely the moment of conception (when the sperm meets the egg) that is important. It is a personal decision whether to include all the other things as well such as timing, etc. One way to include both is to check OPKs leading up to O. When you get a +ve OPK, check what moon sign you will be in when you are likely to O (for me it seems to be 30hrs later, but different for everyone). Then you can decide to BD then and still have up to a 2 day cut off, or to BD right through O.
It also helps you see if you are likely to O close to a change of moon signs. If you are due to move into the opposite gender when you O it might be worth skipping that month as you need to allow a bit of time for conception to actually occur. GL with it all. It is not exactly easy, but if you keep charting your cycle for a while you are likely to see some patterns.