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I seem to recall in the RD-6800 manual there were two methods based on what your largest sprocket was...one was the big-big, and the other was about the line crossing through the jockey wheels being perpendicular to the ground.
Philip_J_Fry-
Yes, that is in the manual for Shimano RDs, for cassettes with a maximum 27T sprocket. It does work in those limited circumstances - a Shimano RD, a big sprocket ≤27T and a double chainset with no more than 16T difference. The thing is, the other method works for everything, so which would you rather learn?
gbj_tester
The inch, which is the amount of extra chain you have to add, is not really derived from the thumb; there is some weak suggestion that the Scottish inch may have been defined in relation to the thumb, but the English and succeeding Imperial inches never were.