I was told my baby was on the 75th pecentile, it turned out she was on the 95th percentile, she weighed in at nearly 10 pound, but she was very long. But her head was on the 50th percentile. I think it is very inaccurate and you should say if you want to go a vaginal birth, you can always go the C-section if needed later.
I know of lots of girls who have had a bad tear, who have gone on to succesfull births afterwards with not a scratch, try to not worry, it will play out however it is meant to. The more worried you get the hard it will be to relax and allow your body to work at it's best. Good luck!!!