Maybe I'm not following you guys, but I'm not sure what the red lines are for. Square rule is designed to deal with timbers that are not exactly the right size. Just measure everything off the reference face, and ignore the opposite side. In this drawing, get rid of the 8" dimension - it doesn't matter. Then show that the back of the brace housings is 7" from the reference face. Just like you did with the tenon height. CB.