The mirror must be brought close to the user's face in order to obtain a
focused reflection.
This generally becomes a plane equal to the edge of the counter. In other
words, you want the mirror to come to the edge of the counter, not short of
it which would cause you to have to lean forward to get a focused reflection.
If the mirror will be mounted on the wall behind the counter, a double
extension arm (see Baci P-2) is necessary to make the reach from the wall to the counter edge.
If the mirror will be mounted on a side-wall, a single arm style is
generally sufficient. Mount the mirror so that when it is in use, it will
come out to the edge of the counter and when not in use it will swing back
and store flat on the wall.