I was reading a TF book the other night about square rule and it said to put the chalk lines at 1.5 which is what the narrow side of the framing square width is. When reading that, I wondered if they meant that a tennon would have to stay inside the talk lines instead of going to the edge of the timber on each side. I'm not sure if that is what was meant, but I don't think so. I think they just want you to use the lines as measuring references to keep things square to those lines. If the distance from the chalk lines to the edges of the timber were not equal on each side, I think then that your tennon would not go to the edge, but rather would be centered relative to your inner timber(chalk lines) and the specified width of the mortise. Correct?