dzeroplus,
The ground effect is a gradual thing, it starts at about 20% thrust advantage (or power decrease, take your pick) at the point where your aircraft just breaks the ground. As you gain height, the effect reduces until by about 1.5 times the rotor diameter it is nil. The line is not straight, the per foot of height effect is strongest at low altitude, and gets weaker as you go up.
Why not try it? On a still morning, take your machine and barely lift it off, with the skids just awash, and note the rpm and power (MP). then lift it to 5 feet and sustain the same rpm exactly. Let it settle at 5 feet, don't pump the collective, let it drift up and down a bit. Note the MP there, should be noticibly higher. Now go to the highest hover the avoid area of your manual permits, and repeat this trial. Much more power yet.
For the record, the effect is not due to a "ground cushion" or "air pressure" or "a bubble of air" under the helo. The ground bends the air flow coming off the rotor, and this bend makes the rotor blades work at a lower angle to produce the same lift. The lower angle needs less power, so we see the ground effect.