A (quick-n-dirty) do-it-yourself method to calculate ebook penetration is to take the 8% *dollar* ratio so broadly reported last fall as if it were gospel and apply a pricing mutiplier of your choice.
eBooks are *on average* cheaper than print books, right? Half the average pbook price? A third? A quarter? Your choice.
Yes, you too can be an ebook industry analyst, just like the good folks at Forester or wherever!
Either way you slice it you'll get an industry-wide unit penetration of 15 to 30%. This, barely three years after Kindle's Oprah moment. 50% unit ratio is within easy reach any way you slice it. It might be a year, it might be four. 2014 is probably conservative, though.