The commercial jigs are waaaay too expensive, and I don't like the idea of drilling a hole in my door jamb. So here's what I did. I drilled 4 small holes near each corner of a square of plywood. After the holes were drilled, I used X's to locate and then drill a 5/8" hole. As a afterthought I located and drilled a 1-1/4" hole along one of the X lines. Oh, and so I wouldn't have to thread the plumb bob line through the 4 small holes, I saw-slotted through the center of each hole. Then I securely screwed the plywood template to the top shelf.
Bob down to the floor and re-create the pattern above. If you use the reflective sticky targets, your pole point won't wander on the bare concrete.
Adjust away.
Dave
Great idea with the saw cuts; I threaded the string through the hole I drilled in the closet shelf. I did it the hard way.