The oft-quoted Calibrating Your Expectations essay at the Alexandrian came to roughly that conclusion, the general consensus here being that it's flawed but fairly accurate, and there are a bunch of posts people have made supporting that general scale (examinations of the CR of Hercules's and Perseus's foes, examples of what you can do with 9 ranks in a skill, etc.) which I can link later today when I get back to my laptop, if you're interested.