OK some comments:
There is really no such thing as IGE as a discrete thing. There is no place where IGE "starts" or stops. The sweep of power gain with lower altitude starts at maybe 2.0 diameters and can be actually measured at 1.5 diameters. If you could hover the body below ground level, in a hole, so the rotor could get lower to the ground, you can get to about 20% power recovery (where IGE hover power s 80% of OGE hover power). From a practical standpoint, the power gain is about 14% for most helos, since that darn body hits the ground first. As an aside, I believe the power advantage would drop, and things get worse if the rotor gets too close to the ground, as the flow from the rotor starts getting blocked by the ground. Never saw any data on that,(called blackage) although lots exists for the tail rotor working against the vertical fin.
To measure it for yourself, try this on a calm morning, with a friend to carefully read the gages while you fly:
Hover very steadily at 1 inch, where the skids/wheels just touch the gound sometimes, dancing if you will. Keep the rpm exact, and note the torque MP.
Rise to about 2 feet and stabiilize and record. Go to 10 feet and do it again. Then 25 feet, then 50 feet.
Now plot the data as a percentage of HOGE power. In other words, if the OGE was 72% torque, just divide all the readings by 72. If the 1 inch power is 62%, then it will come out as 86%, and the OGE will come out as 100%.
There is no IGE on that chart, is there? Just varying levels of IGE-ness.
Oh, Lord, I hope nobody now posts that the pressure under the rotor is what we are measuring, or this whole thread will turn toward the dark side!