I used to think that for example having 9 mm additional jaw width was better than 6 mm. Boy how wrong was I. I talked about this with Dr van der Dussen, and he seems specialized in facial proportions (although he is too conservative - to the point there is not much change with his clients). But he understood the issue of facial harmony. Overcorrection is a problem in PS and exceeding certain estethical boundaries can make you uglier. Sometimes a couple of mm too much can bring someone down from a 7 to a 5.
I give an example of how facial harmony works: a woman with jaw deficit (too steep, almost no angle) wants a chin wing. While the chin wing gives her a jaw angle, it can make her chin area more horizontal as well, because of the drop down of area between chin and jaw angle. This is not aestethical for most women, because they need to have a V shape and not a boxshape. With men, somewhere between V shape and boxshape is the Ideal. Of course, it depends on the face as well. But creating a broad horizontal chin is certainly not always better. Creating a Brad Pitt chin on some people's face can even look ridiculous. Watch out for overcorrection. The best results mostly are the results where the changes are subtle but big enough to be not too subtle (changing nothing). Unless you have a very obvious flaw, then you vlcan go further.
