Hey Dirk, it is such a fine balncing act but I would suggest that it is a combination on both active support/flow and aperture size. I think this is so complex that the sound should determine the “work” involved and the feeling of BCH throughout is necessary.

Maintaining pitch is important of course as is tone purity. If the aperture remained exactly the same when there is less flow, pitch may slip slightly.

Cheers,
Greg