headspace needed to carb keg?

[Ballroomblitz1]

I have a 5 gallon Sanke and was wondering if I needed to have some headspace for CO2. I know this sounds like a dumb question, but can I force carb a fully-filled keg? Do I need to have some space? My thinking is that I should probably have 4 3/4 of beer in my 5 gallon keg, express the air, and then carb. Right or wrong?

 

[IffyG]

I can't speak for a Sanke, but as long as the gas in dip tube is above the beer, I can carb in a corny just fine.