Your blog post makes lots of sense if all you care about is one threshold: contrast must be above X for readability. Then the equivalence-under-monotonic-function makes total sense: you literally don't care what the function does above or below your threshold. This extends to if you pick N threshold points (text must be above Contrast1, contextual borders between Contrast2 and Contrast3, etc.) by first looking at the function and then "eyeballing" those thresholds. But is that the best we can do? The difference between (Ymin / Ymax) and log(Ymin / Ymax) starts to matter a lot as soon as you start talking about "twice the contrast" or "20% more contrast", doesn't it?