A mirror tool used to perform two-point scale bar measurements helps eliminate the need to deploy elaborate rail systems to ensure perfect alignment between the two points and the laser interferometer. By placing the laterally and longitudinally adjustable mirror tool in place of a laser tracker to form a perfectly straight path for a laser beam, tracker positioning is no longer a critical issue. With the laser tracker situated away from the two points at a location convenient for the particular measurement situation, the operator merely needs to adjust the position of a free-floating mirrored surface until the beam from the laser tracker bounces off the mirror and forms a straight line with the two points. Dropping the beam, feared with stationary interferometers, becomes a non-issue.