A correction of fact that does not at all affect your use of the example, Kent.
The ‘centralization index’ is not a measure of tongue position. It is an acoustic measure. It is a measure of a relationship between two frequency-bands of sound intensity.
This relationship of F1 and F2 correlates in an approximate but unmeasured way with tongue height. In all cases, the oral cavity is variably constricted by the tongue to produce the desired differentiation of different vowels, but different speakers control the same acoustic results by non-identical control of the configuration of the tongue (and jaw). Scarcely ever are the positions of the tongue measured in investigations of speech. Sound spectrograms are made instead, and the correlation with tongue configurations is presumed and does indeed follow from principles of acoustical physics.
I’ll put more detail in a place suited for discussion of language.