You wish me to reveal my methods? Very well.
First, as to Lloyd's observation, I doubt that's the case. Any slushpile you look in will have many, many manuscripts that have been rejected multiple times. Sometimes you'll find manuscripts that have been making the rounds for decades. So I believe that PA's slush is just typical slush. That means that around 1% of it is publishable.
But -- PA rejects books. Allegedly they only publish 20% of what they get. Now we know that a lot of those rejections are based on things like length or from being too hard to format, but I hope that they're also rejecting the books that are written by people who are functionally illiterate or actually insane.
That leaves these categories, which
we are told make up 20% of the slush pile:
# It’s nice that the author is working on his/her problems, but the process would be better served by seeing a shrink than by writing novels.
# Nobody but the author is ever going to care about this dull, flaccid, underperforming book.
# The book has an engaging plot. Trouble is, it’s not the author’s, and everybody’s already seen that movie/read that book/collected that comic.
...
# Someone could publish this book, but we don’t see why it should be us.
# Author is talented, but has written the wrong book.
# It’s a good book, but the house isn’t going to get behind it, so if you buy it, it’ll just get lost in the shuffle.
# Buy this book.
I think we can all recognize a lot of PA books in that list, from every one of the described categories.
If "Buy this book" makes up 1% of the raw slush, it makes up 5% of the slush after the bottom 80% is swept away.
5% of 20,000 is 1,000. Hence, my estimate that PA has 1,000 readable titles.