Your math sounds pretty solid to me based on what we currently know. We'll have to see if maybe due to under-ordering if some boxes have more than one single sketch.
I really wish I could have picked up another couple of cases, but it just didn't work out for me. I see on one online retailer's site that the case price has already gone up $15 from the pre-order price.
My sketch collection: http://sketchcollectors.com/gallery.php?user_id=727
Claim to fame: Coined the term BODA