Sym.. the book i have says to start with a cut off 5 days before O. Its hard to know when you will O after getting a + OPK, so i just went off the + being O.. but also how long your LH surge has been around prior to testing. If you dont fall pregnant that month then next month you step down a day to 4 days before O, then 3, then 2. Reason i started at 2 days before O was prior to me getting my Rubella Booster i had tried the 5, 4 & 3 days before O. I was really cutting it fine though as we DTD in the evening on CD 12 and i actually got EW and a +OPK in the morning of CD 14.. so technically it was less than 48 hours between cut off and +. If i was to do it over (if i didnt get a girl this time), id honestly be aiming at more like 3 days rather than the 2 as you really dont know what time of day you will get that + OPK.
You still definately have a chance as female sperm live up to 5 days, male sperm up to 3 days.
Best of luck TTC your little girl.