I think your observations/opinions are basically sound. I think you should also consider quantitating the overall thickness of the profile and most importantly, how and where the thickness tapers from end to end (foil.) You are hitting it with your comments on “pinching” - too much or too far up and you can indeed introduce a tracking tendency. I’d go so far as to say that rail specs on a custom board are easily one of the hardest details to convey to your shaper. Personal preferences are easy enough to identify when you’re riding that favorite board on a good wave - it just feels right. The problem is that everybody has their own specs that work for them in certain waves. “Soft egg” or “hard down” make enough sense to you as you are describing them but a lot can get lost in translation. I know some good shapers who use foam rail templates - nose, mid and tail as they foil out a board. Other guys use rail contour gauges or maybe just grab the rail with their hands. Here is an idea you might consider for measuring rails:
