My recommendation for a girl - I have read and heard several times that girl sperms can live longer, but move slower, while boy sperms move fast but live less. This means:
If you have sex 2-4 days before ovulation, it is a good chance to conceive a girl, since boy sperms would not survive for so long.
If you have sex 1 day or on the day of ovulation, it is a good chance to conceive a boy.
So I would recommend to BD when your ovu test line is getting darker, but not as dark as the control line or even before. If you have regular cycles, I would recommend to measure BBT for two cycles to find out when your ovulation is and then BD 3-2 days before ovulation and no on the day of ovulation.
I think it works, because I have two girls and with the first one I am sure that we had sex only 4 or 5 days before ovulation and that was it that cycle...