Whenever I’m hanging a door with wood jambs, I use a chisel or utility knife to lift a small sliver in the center of the head jamb. Then I use it to anchor the string on my plumb line, as shown in the drawing. The sliver is a quick, handy way to get a purchase for the plumb line, and it stays there until it is eventually hidden behind the door casing. The same trick can be used to hang a plumb line when you’re framing.
Michael R. Sweem, Palo Alto, CA