So I am going to be priming my first keg soon and had a few questions. I know when I fill a bottle, I leave some head space for the CO2 to build up. When I fill my keg, and put the priming sugar in, how much head space do I need? I have 6 gallons of beer ready to go into a keg, but I only have a 5 gallon corney keg. Trying to figure out what to do with that other gallon.