From Google:
Place the bike 25 feet from the wall or garage door (4). Measure from the center of the headlight to the ground with the bike level (5). Now go over to the garage door and mark one line at the same height above ground, and another two inches lower (6). Use a carpenter's level to draw a horizontal reference line (7). Switch on the headlights and mount the motorcycle. The low beam's upper cutoff point should rest right on the lower line (8). Because most bikes have combined high- and low-beam reflectors, setting the height (and checking to see that the beam is centered along the bike's long axis) is about all there is to it. But if you have separate high beams, make the center of the high beam land on the upper line.