Blowouts depend on so many things, but usually it's because you're new/uncomfortable or uncalibrated. Over time, as you approach more, the blowouts should get less and less unless you remain uncalibrated. That's the easy answer. When you're new and nervous, the "blowout ratio" can be quite high, unless you're already confident and socially skilled/practiced.
Instead of trying to find a blowout ratio to compare yourself to others--which can be different one person to the next for a variety of reasons--best to look at where you are now and try to improve your approaches/social skills over time. If you are still getting majority blowouts after 50+ approaches or more, then consider getting a wing to look at what you're doing wrong would be my advice. Hard to diagnose without actually seeing it in person.
Simple/quick suggestions: go in with a good vibe; warm energy, a smile, positive, solid eye contact, and if it's a situation that might make her feel a lot of social pressure (like waiting for a bus, for example), call it out by saying something like, "..I see you're waiting for the bus, so I don't want to take too much of your time, but...", or if she's in a bookstore, "...I realize you're so focused looking for your next book, but I just had to say quickly...". Sometimes people get blowouts because they ignore these things and go in without addressing what she's doing or how she might feel