If you take an average man's size 9 foot, who wears a 25.5-26.5 shell, that fits into a 98-100 last shell as a base to perform measurements on, then what you find is the following approximate numbers:
1) If you measure the flesh area, under your heel bone (calcaneous), not to the edges, but what actually supports your mass, the width of this cross-section is 5cm
2) If you do the same measurement, under the ball of your foot, which is where the midfoot Metatarsal bones meet the base of your toes (Phalanx bones), and measure from the midpoint of the small toe bone to your big toe bone, this cross-section is 7cm
3) Of no practical importance, but interesting, the distance from the ball of your foot to the middle of your heel is 15cm
This Trapezoidal balancing zone that your body uses, 7cm in front, 5cm in back, and 15cm in length (excluding the toes - yes they are important), just happens to have as the mid-point cross-section, a width of 6cm or 60mm - which just happens to be very close to the narrow point 65mm on race-skis (not a coincidence).
I have a number of front side carvers (Volkl RaceTigers, Blizzard GS with Race-plate, Rossi Multi-turn, plus a few more that I share with my former Canadian level racing partner), and I can tell you that in my experience, 67-68 is where the cross over point is from being able to roll the ski on edge, without the feeling of the width creating leverage, because when I use the Rossi Multi-turn, that has a 70 waist, it becomes very obvious that the width of the ski is wider than my brain wants to feel, and I can feel the extra force needed to get it on edge. My soft/heavy snow ski is an Enforcer 88, a ski I love, but I only love it after a few runs where I have to completely change the way I ski - where I develop a somewhat brutalized forced tipping onto the edge with speed type of skiing (it kind of feels like a large SUV just running over everything in its way type of feeling - which is great in heavy crud type snow)
I came at all of this as a lifetime hockey player and someone who only came to skiing much later in life. I had to experiment and buy many different types of skis until I finally figured out that the type of ski that my brain wanted was a race ski that was narrow underfoot so that I could get the feeling that I was on an edge, like in hockey, and I could just turn/carve when I wanted to without thinking - and that is what 65-67/68 feels to me - it's so intuitive you don't have to think and you don't need to expend the energy to 'launch' the ski up onto its edge to carve a turn. I also happen to be a person that badly blew apart my left knee so I'm extra sensitive to the feeling of leverage being applied to this joint.
