But how to apply them to dress is not obvious.
Yes, but there's some variance across cultures in what is considered optimal (varies from .6 to .8 for the ratio you mention).
That may be part of the reason we dress up - but also to embody and advance our own taste, perhaps then for the approval of others, but we don't generally set out from the beginning thinking about what others will like, any more than a musician who writes a song that represents his own creation, and then hopes that others will appreciate it, and by extension, him.
I should clarify again what I mean by rules. Clearly it is POSSIBLE to create a system of rules that will guarantee something that "looks good" to you. A very memory-intensive version would just be to categorize every possible outfit as "looking good" or not. The point of rules is to have something that's simpler to remember than that. So remembering "show some cuff" immediately cuts out all the outfits that don't show cuff, so that you don't have to remember for each individual one that they don't look good. In fact you can think about your own "taste", or your own sense of whether something looks good or not, as a system of rules working in the background, in your unconscious mind, developed by evolution, to evaluate the aesthetic quality of what you're looking at. One way to interpret what I'm saying is that you should trust this system of unconscious rules, and not just the conscious rules you have learned like, "when putting two different patterns together, make them of disparate scales".
If you concede that style is a combination of social construction and personal style (memes and qualia) then we can easily answer why rules exist. It has to do with epistemology. Developing a common vocabulary to describe something allows us to first to share it with others and second to develop our own understand of it. Why don't we always see something and the appropriate aesthetic rule pop into mind? The analogy is this - prehistoric man looks at a rainbow and sees pretty colours and a miracle. Medieval man looks at it and sees seven colours. Industrial age man knows with help of a prism that there are an infinite number of colurs in the rainbow, while the modern man knows all about wave and paticle nature of lights and even the possibility of tachyons. Has the rainbow changed? Has its beauty changes? No. But our vocabulary to be able to understand it and have a conversation about it has changed dramatically. Now the modem man has the option to stay at any one of those levels of understanding; it is still a pretty rainbow! but there will be a select few Who will be studying why light diffracts the way it does, why it the curved shape is pleasing to the eye., why in this variation of the multiverse vibgyor is the sequence of colours - why red does not come before blue, how eye perceives colours etc,
As you can clearly see that there are dozens of posts everyday where the posted clearly knows there is something wrong with the outfit - but does have the vocabulary to explain it. When he posts and asks for opinions- he is engaging in the epistemological process of going from instinct to expression.