There's no denying that some fish need more room, but yes, minimum requirements are arbitrary. Some say 4', some say 6' others 120 gallons, others 180+.
The only barometer that is effectively used is whether the fish, tang or otherwise, is healthy. Regardless of whether the issue might be swimming room, poor diet or poor tankmates, if a fish is healthy, it's fine & if not something needs to be done.
Keep in mind, some tangs are territorial and others are not. Territorial tangs are used to staying in relatively small areas but are more prone to aggression to defend it. Others roam many miles and they do require larger systems. How much larger? Nobody know and that's where the size becomes arbitrary. But remember this: the primary drive to roam, be territorial or aggressive is FOOD not sightseeing. If the food needs are met, the need to roam decreases as does aggression.
Like I said, keep them well fed & keep watch on their health & you will be ahead of the curve.
FWIW, I have yet to see any scientific information of the "anti growth hormone" that causes tangs or other fish to turn inside out. Why would ANY ocean dwelling animal have such a hormone? Especially when in most cases size decreases predation?