Crystal buttons behave differently from covered ones, and the difference decides how deep they are pulled.
A covered button is the same colour as the cloth around it, so it reads as a dimple - the eye sees the shadow it makes, not the button itself. Pull it deeper and the shadow deepens; the effect is gradual.
A crystal button is a bright point on a dark ground, and it reads as a highlight rather than a shadow. Pulled too deep it sinks below the surrounding surface and the highlight disappears into its own hollow; pulled too shallow it sits on the surface and looks stuck on.
So our upholsterers pull crystal buttons to a shallower depth than covered ones - just far enough for the velvet to fold around them and hold them, but not so far that the crystal loses its light. Each is checked from the front at standing distance rather than from above at the bench, because that is the only angle from which the effect can actually be judged.