This below is a test I did a while back, not an actual game, but it's interesting because it's the longest range (over 1500 metres) I've yet seen a flak engage an aircraft, that's getting on for a mile which is pretty impressive as it means flak can easily cover the whole of most CM battlefields..
Note too the flakwagon was moving on 'Fast' while shooting, and it still downed the plane as it's 'kills' screen later revealed!