I Personally don't know the percents, but I have read and heard people say that Hell Week is the first "chopping block" per-say; then Pool Week is the next. People say if you can make it through Pool Week you have a very high chance to graduate BUD/S. This is just people's words, not facts. I'm sure the people up at NSW have compiled some stats and know an average percentage that complete BUD/S after Hell Week.
"What this power is, I cannot say. All I know is that it exists...and it becomes available only when you are in that state of mind in which you know exactly what you want...and are fully determined not to quit until you get it." - Graham Bell