I'm usually good for 1.5 to 2 MOA under "field" conditions. IF your 90% criteria included variable weather conditions, IE not being able to chose when you shoot, then I'd say 500-600 yards. At that distance a good variable wind can easily result in complete misses, never mind just outside 4MOA.
Another consideration is target shape. A symetric round bull or even a square/rectangle makes a major differnce IMHO.
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