Response to Dev control for b/w films

Errm, 18% grey is 18% grey. By definition, it's a neutral tone that diffusely reflects 18% of the light falling on it.
It isn't a density.
The 'claim to fame' of this standard is that light meters are calibrated to give an exposure that's 4 stops up from the nominal 'toe' of the film D/logE curve when metering an 18% reflective surface. What density this results in on film depends on the development given, and only results in a density of ~1.2 (+base +fog), when the film is developed to a bar gamma of 1.
A gamma of 1 is a bit too high for most pictorial work in practise though, and a gamma of 0.8, giving a density closer to 1, would be a more sensible value to aim for.
Anyway, IMHO, what's much more important pictorially is the relationship of all the other tones in the print to this arbitrary 'reference'.

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