It is recommended that a maximum of 35% N1 is used to start the aircraft moving and after this sufficient power should be maintained to keep the aircraft moving at a constant steady speed. Most operators use a maximum of 25 or 30 kts g/speed. Light weight aircraft, as previously stated perhpas need no increase in power to start moving: heavy aircraft may need 35% ! I have used as much as 45% to get a heavy aircraft moving up a slope !