As far as the 1" left to scribe, this is what got me thinking about the whole 2' mark.

The top of the girts in the white barn are dropped 23" from the post tenon shoulder.

I noticed that 23" was an odd measurement for girt layout and remembered the 1" to scribe to fit...

As I read the excerpt from Shaw:

Third, take out the size of the mortised timber on the end of the square; suppose 10 inches to be the one mortised, then 14 inches remain on the square; make a distinct mark at the end of the square, which is called the 2 feet mark.

Therefore, it sounds like mortises and thus timbers were placed at the 2' mark (at least sometimes). Is the tenon length included in the 2' measurement? Sounds like it is. What "end of the square" is Shaw refering to?

I'm confused. But not deterred