CofC's by definition are rather arbitrary in size...the reason being that all of us have different levels of visual acuity. In a continuum of blur, even a single observer might construe a certain blur circle size to be 'acceptable' one day but 'blurred' on another day!
CofC size in the industry assumes a FLAWED and rather POOR visual acuity in an observer of the standard 8 x 10" print viewed from 12" away. Someone with 20/20 visual acuity could look at a photo and say 'blurred' whereas the person with 'manufacturer standard' vlsual acuity would judge "that looks 'in focus' to me".
There is a benefit to having a standard definition of CofC, but we need to realize it is arbitrary.