Quad copter - from PID values to motor power
This seems like more of an aerodynamics question, but I'll take a shot at it.

Maybe what you're talking about is to adjust in terms of percent of THROTTLE? building on your technique this might be

$$OUTPUT = Error * 0.4 * {THROTTLE\over THROTTLE_{max}} + THROTTLE$$

Factoring out THROTTLE,

$$OUTPUT = (Error * {0.4\over THROTTLE_{max}} + 1) * THROTTLE$$

Now considering that $${0.4\over THROTTLE_{max}}$$ is a constant, this is only two multiplies and one addition.



I think though if your throttle output is linear (ie if throttle is proportional to force) the restorative forces to tip the thing back to level should be the same regardless of how high the throttle is set... Said another way, it doesn't take any more or less force to rotate the 'copter no matter how fast it's going.

Hopefully this helps. It's WAAY outside my area of expertise haha