Setting an actual number on minimum altitude for being OGE is impractical. The point of the OGE charts is that it gives you a maximum power requirement for hover based on weight and DA. Wind and ground effect can reduce this power requirement.
The point of IGE charts is they predict a still air power requirement for whatever skid/wheel height is published. It's more of an anticipated power requirement rather than a worst case power requirement.
So where does out of ground effect occur? Wherever you aren't in ground effect. Simple, huh?
Does it happen at 2/3, 1, 1.5 or 2 rotors? Doesn't matter. It's not predicting a power requirement at any of those heights, it is predicting a maximum hover power requirement.
Steve, your low hover is a good idea for power requirements, but there is more to consider. If you have hydraulic problems, SAS/AFCS problems, wind gusts, inattentive ground crew, sneezing pilots, rough terrain, etc. then the extra height gives you a safety margin when the helicopter starts moving. I'd rather use an extra 3% torque and avoid dynamic rollovers or decapitations.
That being said, there are times where a lower skid height is much more preferably. The additional excess torque prior to a vertical departure is an excellent example. Not only do you generate more momentum, but the greater the rate of climb that you have adds in efficiency to the rotor. This is probably due to inflowing air from the climb and aiding the blades to avoid vortices from other blades. With the right conditions, you can actually hover climb with less power than it takes to hover. Unfortunately the opposite can happen in the hover descent.
Matthew.