I keep my plumb-bob string wound around a spool exactly 2 in. long. I drive a screw into one end of the spool, cut its head off and grind the shank to a long, diamond-shaped point. Since I usually work by myself, I can drive the point into the top plate of the wall and dangle the plumb bob off the end of the spool. When the bob hangs 2 in. out from the bottom plate, I know the wall is plumb.
Earl Roberts, Washington, DC