I understand your idea.

I do think you have your work cut out for you to make it real; first in finding, then in mapping the shoulders.

Personally I doubt that you find a clear cut shape or threshold that defines shoulders in general in a meaningful way.

Consider D3200, it has a long arc of a shoulder, depending on the CI you might consider "normal" the shoulder can even make up most of the curve. XP2's curve is similar in shape. Like the energizer bunny they just keep going and going.