I clip both wings, and mine can't fly over a 3' fence, and they're a heavy production breed. Before wing clipping they were capable of flying up at least 5' onto the coop roof. I tried clipping just one wing and it didn't make any difference; they still flew up onto the coop, just a bit less efficiently. Their feathers on one side still had lift, enough to compensate for the clipped other side.
The thing about fence is the top; if it's thin wire it's really difficult for chickens to get a perch on it. Chain link and the top bar of chain link is really easy to grab onto and haul themselves up onto, clipped wings or not.
I have my 1/3 acre backyard surrounded with 2' green coated rabbit fencing. I've never had a chicken fly over it. I think it's because they really can't see it very well again the backdrop of grass and wooded area. Maybe this winter it will be more visible, but they tend to find places to squeeze under or between the house and fence rather than fly over it.
I go into their coop after dark, pick up a snoozing bird, hold it close and extend a wing, then trim an inch off all the long feathers on both wings. It only takes seconds per chicken and doing it after they've gone to roost eliminates chasing frantic chooks all over the yard, sliding in poop, potential for injuring a chicken, etc.
