Drag the lens to the center of the printed circuit board (PCB). The lens is the small blue half circle. The PCB is moving horizontally below the lens. Each time you mount the lens, the location error will be updated on the right side of the screen. The location error is a measure of how far from center the lens is mounted. Negative errors are left of center with positive errors being right of center. The LSL = -20 and the USL = 20. The button in the lower left turns the PCB center line off and on.