heedm,
The spinning mass does not counter the steady torque of the rotor, it cannot, except for the tiny fraction of its power that is lost to friction. The wheel will only balance the torque while the wheel is being spun up, which is where it makes its pay in the screwdriver. It counters the twist jump (strating torque) of the tool when the tool is turned on. It cannot counter the running torque of the tool against the work (such as when a sanding disk is being pressed against a wall).
Your calculations would work to counter the torque of the rotor during rotor run-up, but not during normal powered flight.
EDIT I noticed your post after I wrote this, heedm. You are right on. One of the major problems with helo development was the fixation on torque balance via counterrotation in the early days. The simplicity of the single rotor helicopter (and NOTAR is one, too) usually outweighs the slightly greater efficiency of the double rotor (tandem or coaxial) designs that compete.
The tail rotor uses about 3 to 6% of the main rotor power in a heavy hover, and almost nothing in cruise.
[ 30 November 2001: Message edited by: Nick Lappos ]