What ball? 
the center of my swing is in front of the center line of my body - so I set up there (square body and alignment), I look there and sometimes even defocus a bit and just think about my takeaway and keeping my head still once I start my backswing
For a Ball centric answer:
(driving, I guess that means I look about 4 inches behind the ball)
(fairway woods and long irons - pretty much it'll fall somewhere on the ball depending on which club - back edge of the ball for the woods, middle to front of the ball for the 4i/5i)
(the other irons - focus is in front of the ball (ball on inside of right heel), and even if I can't setup in front of the ball with actual head of the club, I do try to set up my shoulders to that point and swing like I did)
Pretty much the same swing (allowing for different length clubs it'll refine somewhat) for any of those clubs. And in each case, a good swing will always find any ball that I happened to find in the path of the club head. (make a nice swing rather than "hit the ball")
I'm getting a LOT of consistency (repeatable distance and straighter shots overall) since I started doing this (learned it from a video on driving I read on these forum pages) - handicap has dropped from a 15/16 to now under 11 in a VERY short time.