Having been an engineer, I seem to remember from school days that it was N for number of turns, as in rotational speed. As any given engines' RPM limits happen to come out to a convenient number, %RPM was introduced for ergonomic/display reasons. I guess if they'd wanted to, one could have aircraft airspeed indicators marked in % of Max Speed also.
Last edited by 727gm; 10th Aug 2009 at 20:11.