I am not even sure if being close will get you a proxy "kill". I watched one guy attack a flight of buffs pinging the leader before breaking off to set up another attack. In the meantime the pinged leader dived away leaving his drones behind which promptly exploded! - I called the attacker up who was still nearby and asked if he got the kills. He said no, only the lead plane which he had by now finished off.
It seems to me that to get a proxy i.e. a kill without hitting the enemy, you need to be in some sort of attacking attitude. Such as when you chase some-one down without shooting and they crash in front of you. That in my view is a valid "kill" anyway.
Accidental proxies - like when some-one augers nearby and you were not actively attacking are IMO just a bit of luck. As for how close they need to be, I get the impression not more than say 1000 yds - Just my guess as I don't recall seeing distant proxies.
I must admit I nearly alway try to put a few rounds into the drones when attacking a flight, especially from side on attacks when I can "rake" the flight. I tend to go for the drones first when rear quarter attacking. Point being, if you kill the leader and he does not bail and you have at least pinged the rest of the flight, the drones will also become kills. - Sorry its a bit Gamey, but I don't like them getting away for nothing! - And I am pretty sure I have been on top of drones that have exploded and not got a proxy (Kind of annoying when you are just about to pull the trigger!)