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.
Collecting: money in the bank for some projects around the house | hope to be back picking up sketches in 2015