3) Mark point a1 on the wall, in the middle of the horizontal line
at the left edge of the horizontal line (see figure 5).
Figure # 5
4) Turn the laser level counterclockwise until the right edge of
the horizontal line reach's near a1, mark a point a2 on the wall
in the middle of the horizontal line (see figure 6).
Figure # 6
a1
x
x a1
x a2
1.5 m
17