Describing an uncertain face outline

A claim that face categories have obvious boundaries has become the topic of our next discussion. I am skeptical because a slightly longer outline does not suddenly become a completely different object. I would like to compare the reasoning behind two labels applied to the same borderline example. We might start with separate observations of facial length, cheekbones and the contour of the chin. One possible starting reference is face shape calculator for the vocabulary involved. The reference would provide a starting point, while the reasoning would still need to be explained. Using one unchanged example would keep camera angle differences out of this particular discussion. How would you demonstrate an uncertain boundary without simply dismissing the usefulness of categories? I want the discussion to leave room for useful descriptions as well as honest uncertainty. Participants should explain which visible features support their interpretation.