Have you considered defining the ranks based on the job responsibilities rather than the backgrounds of the applicants? e.g., "will need to do X without supervision" / "will need to supervise more junior people" / etc?
Along the sames line, if there is a variation, then write it down as the norms or minimums, at a minimum, they will be able to do 3 out of this 6 things, etc.
Comments
Have you considered defining the ranks based on the job responsibilities rather than the backgrounds of the applicants? e.g., "will need to do X without supervision" / "will need to supervise more junior people" / etc?