I was really intrigued that one book mentioned the Flesch Reading Ease Score which is defined by the equation:
A sign that this is a "widely accepted" metric is that it is incorporated in Microsoft Word.
The main thing that bothers me is the number of significant figures in the coefficients.
But also, surely you could devise the metric so that it actually does give values in the range 0-100, like most guides claim. Pathological text can produces negative values or values greater than 100.