18% is the midpoint for averaging meter use, and esp applicable to color film. But how many stops
of range with a black and white film the Zone Systems applies to either side of this really depends on the specific film and developer. I once took an entire stack of gray cards from several mfg and
read them on a high-end spectrophotometer. None of them were 18%, and all of them varied with
the exact wavelength of light because none were truly gray.