Reference for the "Elbow length" method?

[kno10]

The documentation of the elbow package suggests this is based on the reference Thorndike 1953: https://github.com/annoviko/pyclustering/blob/bf4f51a472622292627ec8c294eb205585e50f52/pyclustering/cluster/elbow.py#L4 https://github.com/annoviko/pyclustering/blob/bf4f51a472622292627ec8c294eb205585e50f52/docs/citation.bib#L552-L556 Yet, I cannot find the "Elbow length" equation used in this reference, in fact he appears very skeptical that such elbows can be reliably identified (for a good reason...). Is there another reference for this particular method?