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Originally Posted by B767Driver  AC120-91 Airport Obstacle Analysis suggests course guidance for LOC/VOR/NDB/ARC navigation. The limits povided are basically the 5/5/1/2 degree parameters. |
Just skimming through the AC, it's intended for departures, as you said, rather than arrivals, to make sure you can maintain the Part 121 and 135 obstacle clearance requirements. The course widths appear to be given to tell the carrier how wide of a course they must consider for obstacle clearance.
Without reading the whole thing, it looks like they're just making some assumptions about the likely worst-case scenario of needle deflection, rather than making a recommendation. (Although the prudent operator would fly within those assumptions!)