What I have done (rightly or wrongly) is fill the keg up to the shoulder after batch priming and cap the keg with a lid with a pressure release valve. Leave for 2 weeks then take the lid off and attach the tap and regulator.
I don't purge the keg before filling and have to lift the prv a couple of times to take off the cap before attaching the tap.
By leaving it to condition, the beer in the keg carbs up itself from priming sugar. I have found I can dispense a 5 litre keg with 1 co2 bulb doing it this way. But, as I said, I'm still experimenting and open to suggestions