and make a pencil mark where the beam falls.
rotate the laser, but do not move it from its position, and make additional marks at the three other corners in the room.
measure from the first mark you made straight down to the floor and subtract 1 inch. for example, if it’s 4 inches from the beam mark to the concrete floor, subtract 1 inch to arrive at 3 inches. this is the control measurement that you will use to adjust the other corners.
go to each corner and measure down from the laser beam marks the distance of the control measurement and put new marks on the wall. in our example, each 