Holding a tape measure in one hand and pencil in the other may not be an option when placing that picture hanger in just the right spot.  If working alone I will use the tape to measure my desired spot, stare at that spot while setting down the tape, mark that spot lightly with a pencil, then measure to see how close I got. Remembering how far off I was and in what direction, I will adjust by marking a new spot and erasing the old one then measure again.  May take awhile to get it exactly where I want it but it gets there.  I'll use the same method with a tape measure, level and two marks to get a level or plumb line.

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