The trick is to fill the left over space with co2. This goes for any time you keg....
Purge the keg with co2 before adding the beer, by putting the gas in line on, turn on gas to 10-15psi, let it fill (listen), pull the pressure release valve on top of the keg for a couple seconds, repeat twice.
Then gently remove the lid the top of the keg and siphon the beer to the bottom trying not to disturb the cloud of co2 so much.
Once full, button it up and purge again.