There's no set BF % to get that 6 pack, we're all different, and it depends on where you hold most of your fat, and where your fat comes off of when you reduce it. Not sure if what I'm saying makes sense.
For ex. if you hold alot of your fat in your tummy and it's the last place it comes off of, then you will probably have to be really low in BF% to see that 6 pack.