You could tie it to the programmer's competency matrix if you want to make it a "harder" metric. Titles are not set in stone; it's all HR speak. Convenient buzzwords to encapsulate characteristics that may not even be precisely defined if they are defined at all.
To tie it to the PCM:
Junior Developer: Rates at least an n^2 in most disciplines. Could rate n or log(n) in a few and can have a few 2^n and still be okay.
Middle Developer: Majority n with a minority of n^2 and as few 2^n as possible. As many log(n)s as possible but will likely still be a minority.
Senior Developer: Majority n with a strong minority or even slight majority in log(n) and as few n^2 and no 2^n.
Legendary: Log(n) in everything possible with a few ns and nothing below that mark.
Specialist: Log(n) in their specializations and experience appropriate otherwise.
Obviously these would be "ideals" for each category. I doubt there are many that would fit the title who would truly measure fully to the standards.