Gary,
I actually have mine a little forward of the "center" mark. I just set the toe piece a little forward and adjusted the back. I do the Ball of foot to center of running surface. The first couple of times it was a real PITA.
I wasn't sure what I was doing and wasn't sure what corresponded on my boot to the place where my ball of my foot was supposed to be (on a sneeker you can just push in and feel - not so with a ski boot). Also, if you are a little perfectionist like me, where to the millimeter is the CRS?

. Where do you measure on the shovel and tail? Gotta make sure you get it right....right?
WRONG - too much hassle. Now I pretty much guestamate that if you measure along the side and take the midway point between where the widest place on the shovel and tail is; then you are close enough. I have a place on my boots that is also my best guess. Now I can get that done pretty quickly. With the railflex bindings, I just adjust up and back if needed on the sole length toe and heel pieces and that is my starting point. Then I go up and back depending on conditions. For me BOF-CRS is pretty sweet for a typical groomed run. Depending on the ski, it can be 1.0 - 2.5 cm forward of the mark on the ski.