After I established my point of origin (the corner nearest the camera) I began setting the blocks. I had to draw lines (with the transit level) to establish the north line and the west line. This has everything lined up with the back of the existing house, then 90º towards the existing rear. So far, so good.
Next up, I had to set the beams. They had to be leveled to the existing floor inside. I went all the way around the house with the ledger beam to keep things level with the existing floor. Guess what, it all came out level.