Hi Cece, as you get closer to O it is often a good idea to two OPKs a day, one around 11:30am and the other around 6-7pm. You can get the odd cycle where the surge is so quick you can miss it by doing one a day.
Having EW can mean O is about to happen but also can be several days before O. The common/layman thinking on that is that sperm can live for up to 5 days (or longer) in fertile fluid so therefore you want to have some beforehand to increase the odds of survival. That way when you do finally O you have a few still surviving.
I watched a documentary (I have the link if anyone wants it) and it shows how it is necessary for teh sperm to spend some time in the fallopian tubes as there is an enzyme produced there that they need as 'armour' to help penetrate the egg. It really is quite interesting how it all happens. no wonder life is such a miracle.
So long as you can BD every 36-48 hrs from just after AF then you shouldn't have to worry about timing or signs. Some experts reckon that 36 hrs is optimal.. but on FF software when you do that way it gives a pattern like my chart this month and FF only gives a 'good' rating. It doesn't know that BD was at midnight and O was around 2-3pm the following afternoon so about 15 hrs... TMI probably lol. I know we should have BD once more Sunday evening but honestly I was too tired and thought after all these months it hasn't happened so not likely it would make any difference so I rolled over instead and went to sleep.
hugs all
Judy