In clear, flat open space, I would think -65dB at a 50m radius (100m diameter) is pretty close to what you should see. Note that the Rx sensitivity will dictate what the clients ACTUALLY see, as well as whether the client is 1-stream, 2-stream, or 3-stream. So you may have a client at 50m distance who sees -65dB, but another that sees something better OR worse.
But 50m radius I think is a good ballpark for *flat, clear line of sight, no obstructions*