How does one measure this? Center of toe and heel pieces centered to ski?
That's not hard to do at all with simple shop tools. You can use a Vernier Caliper if you have one, or go low tech and just use two carpenter squares, some painter's tape, a metric ruler and pencil. Mind you, you will only be as precise as you have the ability to be with either high or low tech tools, so if you don't have an engineer's mind and/or a mechanic's feel then, buyer beware, or in this case user beware.
Low tech method: ( assumes a "flat" ski, not a system rail mount )
Note: This is the center width line running longitudinally, not cord center line, and not of the entire ski length, just the binding mount area. Let's assume you are using a paper binding template and want to center it before you drill. You will need to find the center line of the ski at each ends of the template to line up the center line of the template to the center of the ski. You could also use this center line to verify that a mount jig is centered. This assumes you've already figured out the boot center mark and binding center marks. That's another task not covered here. We are only covering the ski center line longitudinally.
First off, ignore the top sheet and any marks on it because top sheet marks may not be correct (we can debate how that happens during manufacturing elsewhere). Your goal is to find the center of the ski, based on the width at two exact fore/aft locations, at the ski's base, metal edge to metal edge, and transfer that to the top sheet. You are interested in the width of the ski at the surface that interacts with the snow, not the top of the ski. Yes the center should be the same top and bottom regardless, but as they say: shite happens and this will verify it if nothing else.
Find two spots on the ski, one just fore and one just aft of your binding template extreme ends. Note: the width of the ski will be different at any given location along the ski length. Place a piece of painters tape across the the top sheet of the ski at the template extreme locations. Let's start at the fore (towards tip) location. At your painters tape fore location put a carpenter's square body flush along the bottom of the ski and the square's tongue against the metal edge. Make sure the square is not cocked sideways and is as perpendicular to the ski edge as you can make it. Place another carpenter's square opposite the other on the other side of the ski. Again, make sure everything is flush and not askew. You can now simple measure the distance at that exact top sheet location between the two tongues of the two carpenter's squares using a good metric ruler and mark the center of that measurement on the painter's tape affixed to your top sheet. Repeat at aft ( towards tail) location. Now you have two center marks on the top sheet (based off the ski's base width) on the painter's tape at these two extreme binding end locations. Take a straight edge and draw a line between those two center marks longitudinally. Be mindful of the pencil line width. Now you have a center line of the ski within the range of the binding. Again, this a low tech method. Anyone can do this with enough care and some simple tools they probably already have around the house.
Edit to add: Oh darn I got side tracked. I wanted to add the reason you need to measure the base width edge to edge and then transfer that to the top sheet is that measuring the top shoulder edge of the ski is problematic, esp. with cap ski construction. Jigs essentially do this.
