dont get a tape measure and put your bindings half way between the tip and the tail.  unless your skis are designed to be mounted that way, they will ski like shit.  most skis have alternate center marks (k2, 4frnt, etc...). if you have your heart set on center mounting, dont go any further than an inch forward of the center mark on the ski.  like i said before, if you go any more, they will ski like shit, and you just fired a bunch of holes in a brand new pair of skis that are pretty much useless now.
if i said it, it must be the truth!