The blog post is pretty out of date now. We had too many issues relating to seams at the edges of those sections, that we just removed that mode and basically render it tile-by-tile now, so it doesn't matter what the tile content is. We're also going to change it again in future. So I wouldn't try to optimise your tiles based on any current algorithm at the moment.
Empty tiles are simply skipped so don't use any fill rate.