Replacing "size" with "linear dimensions" doesn't change anything except for making the language sound sort of "scientific." Why do you use that dimension rather than area or volume or, as stang151 suggests, maybe none of the abovel? And of course everybody knows what "1/4 scale" means, but that's a convention, not a scientific principle. My impression of what models look like in flight is the same as stang151's. At any reasonable distance, they all look about the same, perhaps because real depth perception (the kind you get from binocular vision) doesn't work beyond a few yards distance. So perceptions of "scale speed" shouldn't depend much, if at all, on the distance between the observer and the model. And, even if distance is relevant in some ways, the relation shouldn't be linear. Which would mean that calculations of fractions, whether of length, area, or anything else won't give us an answer. That doesn't mean that "scale speed" in the sense of what looks "right" to an observer is meaningless: a model of a cub (any scale) tearing down the runway at 150 mph would look ridiculous, as would a jet floating in for the kind of landing lightplanes make. But it does mean that there isn't any one answer like "X percent of full scale speed" that scale modelers should aim for.

So I guess my bottom line isn't so much that you have arbitrarily chosen one measure of scale without explaining why, but that this isn't a question that can be reduced to any kind of formula. It's art, not science.