Obviously, proportions that are aesthetic are mathematical
so why do designers still do guess work? can we not turn to science to decide the proportions of whatever we're designing? architecture, cars, paintings, etc.

Is there a book or resource that takes the guess work out? Like I have something that is x length, and the next feature is going to be smaller, so I put that length into an algorithm and get some options for the other feature's length to choose from?