Reference for the "Elbow length" method?

[kno10]

The documentation of the elbow package suggests this is based on the reference Thorndike 1953:

pyclustering/pyclustering/cluster/elbow.py

Line 4 in bf4f51a

@details Implementation based on paper @cite article::cluster::elbow::1.

pyclustering/docs/citation.bib

Lines 552 to 556 in bf4f51a

	@article{article::cluster::elbow::1,
	author = "Thorndike, Robert L.",
	title = "Who belongs in the family?",
	journal = "Psychometrika",
	year = "1953",

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?