I'm just going to through this out there, you are probably thinking way too much about the distance between your hands. There is no correct distance on close grip bench that you need to aim for. The main thing is just that your grip is "closer" together when compared to your normal bench. As your hands move closer together the focus shifts away from your chest and more towards your triceps.

I have seen MANY people have rediculously close grips with heavy weights on this exercise and all I can say is that they are eventually going to mess up their wrists. In my opinion 6 inches is WAY too close.

If you notice in the video posted by Hrdgain that guys grip isn't extremely close together which is why he is able to hit this exercise so hard.