Your definition of where the "knee" lies may not be mine. You need to define carefully what that point is, and even then I think you may have problems to find that point consistently.
My point is, unless you can define (mathematically) what the knee looks like, you cannot program an algorithm to do so, and if you get some code from someone else, their idea may not coincide with yours. (Yeah, while even a supreme court justice says that he knows "it" when he sees it, writing a computer algorithm is not quite so clear.) So expend some thought, and define carefully that location to be found. Once you do, the algorithm is often trivial.
