I wonder what that figure of 1.3% actually *means*. It's a strange way to
specify what is really an angle between the horizontal and the beam. I
presume that 1.3% equates to arctan(1.3/100) or 0.75 degrees.

Certainly when measuring road and railways grades the percentage is either
the tangent or the sine, depending on who you ask. Consensus favours the
tangent as being more correct but with the sine being used in practice
because it can be easier to measure and there is beggar all difference in
practical terms for slopes shallower than around ten to fifteen degrees
anyway.


I suppose it's
done like that because the easiest way to check the setting is to position
that vehicle a known distance (eg 10 metres) from a vertical wall and
measure the vertical offset (eg 1.3% of 10 = 0.13 metres) of the beam from
the straight-ahead position of the headlamp.

Seems reasonable and also is probably correct.