One thing with the line-of-best-fit is that even though people can pick different data points when they draw the line and all be "right," they will all be fairly close together. That fits what I have been saying about the changes to the fluff being okay as long as they fit the general theme of the class, so in that context I agree that line-of-best-fit works.
Can a gnome be tribal? Yes. Can they be wild and uncivilized? Yes. Are the mountains outdoors like the other terrains mentioned? Yes. Gnome barbarians are fine as they can fit the general theme of the barbarian class. A cultured knight who has a bad temper does not fit the general barbarian theme and the line-of-best-fit would not apply to that. Again, that's not to say that such a character is bad or wrong. It's just not the barbarian class, even if it uses the barbarian class mechanics.