In the course of writing about the "fourth highest of five levels", I looked around at how the pattern "Nth Xest" is used in general. I found that uses of such expressions overwhelmingly count from the "top" where X names a top-oriented scale (high, big, long, etc.), and count from the "bottom" where X names a bottom-oriented scale (low, small, short, etc.) In other words, unsurprisingly, "Nth Xest" normally counts (up or down) from whatever end of the scale "Xest" names.
Another (less logically necessary but still unsurprising) thing I noticed is that top-oriented counts are always a lot bigger than corresponding bottom-oriented counts, and that counts decrease almost-proportionately as N increases. Thus from Google Books ngrams: