I have been using OV predictors this month and I OV'ed on day 11 & 12, we BD'ed on day 12 in the evening and then BD'ed on day 13th, Im now on day 14, would it be worth BD'ing again? What is the window of opportunity when you 1st detect a LH surge - is it 24 hours, 36, 48 ????? 
