in measuring both ways at the same time the laser adds together BOTH of those measurements to give you the total length. for example, if my length that i want to measure is 12 feet exactly, and i place the laser 4 feet from one side, the laser measurement of the other side AUTOMATICALLY and INTERNALLY becomes 8 feet . one of the lasers will determine the length in regard to the direction it is pointed TOWARD. the OTHER laser beam will measure the poinf that IT is measuring toward, there two measurements will "collide" with each other at a point INSIDE the laser and the instrument will then INTERNALLY, AUTOMATICALLY add the meaurements and give you the actual length......it has nothing to do with any kind of placement at any centering point on the exterior or interior of the laser unit itself.