Look at the instrument rating PTS for acceptable standards. I was told by an ops inspector that is the only place the FAA publishes what it considers "established" And even then its by inference and not by clearly stated regulation(if there is such a thing) I guess the reasoning goes that these are the standards that a prospective instrument candidate must perform to, so they must be binding to other than students. Clear as mud isn't it?
This was a number of years ago I spoke with the inspector, so things may have changed. FSDO's are pretty good about answering questions, give 'em a call. Shop for the answer you want. If you don't like the answer you get, call another FSDO.