pcpahari_IAF,
You mix the two effects a bit, they are quite different.
Transient droop of the engine/rotor is due to the lag of the engine in responding to collective pitch. It is measured by noting the minimum rotor speed as experienced when a power demand is made in a specific period of time. For example, a pull of collective from 0 torque to 95% torque is made in 2 seconds, and one sees a minimum rotor rpm of 5% below the normal (a pretty fast response). The collective pull is made by carefully trimming the collective pitch at 95% torque and noting its position, then lowering the lever to get to zero torque, then raising it smoothly and steadily to the previous position in the agreed upon time.
The main ingredients in transient torque response are the engine's maximum acceleration rate (usually described as the N1 rate, which should be perhaps 10% per second or more) and the rotor inertia (which resists the power demand and damps the minimum rotor rpm.) The engine is protected from stall by slowing its N1 acceleration, so stall protection balances against engine responsiveness.
Static Droop or Permanent Droop is the long term reduction in rotor rpm with increased power, usually about 1/2% per 10% power. This is put into the old-style governors as the best way to make the engine stable because it does not allow the engine to oscillate about the set rpm. That rpm "hunting" or oscillating about the set point is what we call rpm instability. Think of a thermostat in a cheap house, one that lets the house cool down to 18 degrees before it turns on the furnace, and then allows the house to go the 20 degrees before it shuts the furnace off. This house temperature constantly oscillates between the two temperatures because the thermostat is not stable. If one makes the thermostat shut off at a lower temperature on the heat cycle, the temperature stays closer to the set point, and is stable. In a similar manner, a permanent droop was necessary in old governors for stability. Usually, the collective bias would reset the selected rotor rpm upward with increasing collective pitch, which would reset the rpm upward to cancel the permanent droop - making it invisible to the pilot.
In the previous paragraph I say "old style" governors because modern digital governors have better control logic that generally removes the need for permanent droop - these modern governors are isochronous, constant speed.
Here is a fine discussion of droop from Woodward Controls that has plots of rpm vs power for transient and static droop (they make engine fuel controls). Note that it describes electric generator controls, so they use 60Hz as a speed reference. Same theory, of course:
http://www.canadiancontrols.com/documents/technical/Speed Droop and Power Generation.pdf
Here is a larger, more general discussion of how the fuel controls actually work, it is fantastic (although dated, as most FADECs use computers, not springs and things!):
http://www.woodward.com/pdf/ic/26260.pdf