Step 7. Tape sheets of paper above and
below the point of intersection on the wall. With a carpenter's square
draw a vertical line through the point of intersection that is perpendicular
to the horizontal line.
Step 8. You now have a target of perpendicular
lines that have their point of intersection located where the laser beam
intersects the plane of the wall at right angles. The horizontal line
is already lined up with the RA axis.
