Great thread.
Another way to use a combination square to get the exact center line:
Put a strip of masking tape down middle of the ski - just eyeball it. It's gotta be long enough to span the whole length of the binding mount area - longer is fine, too. Put another strip or two down on top of that one - you'll see why later.
Take your combo square put against the ski edge with the "ruler" part going across the ski top. Yes - the ski edge is not parallel to the center line, but as you'll see in a bit, it doesn't matter.
Take an x-acto knife - place its edge against the ruled egde of the square, somewhat near the center of the ski. Use one of the ruled measurement marks as a reference. Many combo squares have those rule markings stamped into the edge, which helps you hold the blade there. Holding the tip of the blade agains the ruled edge of the combo square, and ever so slightly biting into the masking tape, pull the two together down the length of the masking tape. You're scoring a very shallow line into the tape down the length of tape - and it's only somewhat close to the actual center line. This will be a line that effectively parallel to the ski edge (an arc that matches that of the edge). This is why you use a double thickness of tape - to allow you to comfortably score the tape without marking the actual topskin.
Now flip the square over and do it from the other side - putting the xacto blade on the precisely corresponding measurement mark on the opposite side.
Unless you happen to eyeball the exact center when scoring the lines (unlikely) you'll have two lines scored into the tape. Those two scored lines exactly span actual centerline of the ski. If they're close enough, you can then split the distance to get the true center line. If they're somewhat far apart, repeat the process moving the xacto blade a bit closer to the center line. If you repeat it once or twice, you'll get two lines that pretty much tell you where the exact center line of the ski is.
With this method, you're not actually measuring anything - you're accurately bisecting ski to get the exact center line.
An even better tool than the combo-square/xact knife setup is adjustible wood scribe - but I'd suspect most of us don't have such a thing - but they're not hard to make, either.
Finally, if you don't mind somewhat sacrificing a combo square (they're cheap) - you can file a little notch into the ruler edge to exactly hold the exacto blade in the same spot when you flip it over. This works better if you want to use a pencil instead of an xacto knife.
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