Obviously upon seeing this post, the first thing I did was go find out how many of my fics fit this pattern. Out of 44 fics marked explicit on AO3, I had 22 within those word count parameters and nine I would tentatively consider "porn" as opposed to "fic with some explicit sex." And out of those eight I had five that fit the ratios. Of the others, three were 55%+ porn, and one had two stints of porn with a long clothed conversation in between.

(I also had trouble because I wasn't sure if some of the other fics counted as porn; see poll.)

I don't think this says anything about the validity of your ratios; mostly I think it speaks to the fact that I don't write as many outright PWPs as I thought I did, and that I care more about the relationship development than th actual sex, which I already knew. Also of the nine fics, six were a/b/o, lol @ me.

I'm going to be thinking about your "how did we arrive at these ratios" question for a while, though. Obviously you don't have to have them! Your porn can consist of a much higher fraction of the word count than 30%, and lots of fics do. Yet I immediately knew the kind of fic you were describing. Hmm. I think to some extent it's something we've all learned together, right? A formula for storytelling we all soak up without even noticing, this generalized shape of story arrived at by averaging lots and lots of fic.