The little gadget shown in the drawing is a device I learned about many years ago when I was doing production door hanging. None of its dimensions are critical. I kept mine small only because it takes up less storage space. It is symmetrical, so it works with either end up—one less thing to think about.
To use the standoff, assemble the three pieces of the jamb, but leave the head jamb and the strike jamb loose from the framing. Tack the top of the hinge jamb to the framing. Attach the standoff at the top of the hinge jamb. Move the bottom of the hinge jamb toward or away from the framing, as necessary, to make it 11/2 in. from the plumb bob. Shim and nail off the hinge jamb. Use a framing square to set the head jamb square to the hinge jamb.
If hanging multiple doors of the same width, cut a piece of wood the length of the inside dimension between the hinge jamb and the strike jamb. Use that piece of wood on the floor between the two jambs to set the strike-side jamb. Finish with a few more shims. It’s simple, accurate, and relatively fast.Jim Stuart, Glendora, CA