I don't have the text in front of me, but if I recall correctly, Adams is talking about the viewing distance at which the perspective would look natural, which is not necessarily the best or expected viewing distance. For instance, if you look at an image made with an ultrawide lens from a very close viewing distance, the perspective will look natural, but if you look at it from a more typical viewing distance, it will appear exaggerated (i.e., things will appear to be farther apart from each other than they are), as the photographer probably intends.