和 this is the Short (Back) Wall, 1 required. Note that this is pretty minimally framed. The Blocking should be flush with the outside edge of the 2x4 framing, 和 the CA Corner should be flush with the inside edge of the 2x4 framing. Toe nailing of the blocking is just fine, its providing a nailing surface for your siding to run horizontally. You can stagger then vertically an inch or so, if it makes it easier, you've plenty of width to play with.