One of the quickest and easiest ways to plumb up or down from any given point is with a plumb bob.
We're using the plumb bob to transfer layout marks from the floor to the ceiling.
Suspend the plumb bob about 1/2 in.
above the floor and center the point exactly over the intersecting lines.
Then mark the location on the ceiling.
The key to accuracy is to wait for the bob to stop swinging.
To speed things up, ask a helper to steady the plumb bob while you adjust the position of the string. Plumb bobs have one major drawback: They don't work in windy conditions.