The questions
Footprints raises are important. Yes, although all
women are different, these differences are
specific and can be considered to be
data. There are still general rules that can be applied (taking the above data into account) to come to a correct
solution for each
instance.
Similarly, every pianist has, due to genetic variation, somewhat differing physiological structure. This can mean that the various joint angles can differ somewhat from individual to individual. However, there are two things to consider here: (1) there is a range beyond which the adjustment of, for example, wrist height will no longer yield appropriate structural integrity; (2) what's more important than pure positioning of joints is the underlying muscular state (which is invisible and evades detection by merely observing the subject).
Furthermore, an individual may be able to do something incorrectly and still attain relatively high performance. However, in this case, some other aspect will suffer. Often, this manifests itself in physical injury at a later time. Remember, "proper" technique implys the meeting of more than just one or two criteria.