I would characterize it as half full or so.

You have a rack with a piston in the middle of the body and a seal on each end. Those 3 seals must be kept lubed to slide without tearing up. 8-12 ounces of fluid in there ought to be enough. You really don't want it solidly full because then you have to fight the resistance of the fluid tranferring both ways in the hose as you operate the rack. The hose with no power on it simply seals in fluid and then allows it to go from side to side while bypassing the piston. If about 50% air then the effort to work the rack is easier for you. All you need is enough fluid to lube the seals and gears, working the rack splashes it all around in there, even more so with air mixed in it.