I think if everything is done right....that number approacjes 100%. By everything right I mean: 1) adequate age and weight on the female, but not too much weight 2) mature agressive male 3) proper cycling of temps (and I even manipulate photoperiod, although I understand the the natural range of the BP, being much closer to the equator than we are, has a less dramatic variance in photoperiod) 4) TIMING
In the past 3 seasons, I tried for 10 clutches and produced 9....the other female slugged out. I know from the following season that the one unsuccesful attempt was due to bad timing on my part (female just naturally goes later than all my others, so needs a male later).
Several larger breeders use an ultrasound to make sure to place males with females at just the right time.