If you are patient a good sealed system will self purge the air out in a few cool/heat cycles. Technically it does not matter what the level in the over flow is as long as the bottom is covered in coolant so as to not suck up air on cool down. When the system is dead cold the radiator should be 100% full.
As the system heats up under pressure, any air in the coolant becomes dissolved in the coolant. As it is pushed into the overflow, which is not under pressure, the air undissolves. I've found replacing the overflow tube with clear tubing shows the coolant in the hose so you know the radiator is full. If there is air in the hose there is air in the coolant and more than likely some sort of minor vacuum leak on cool down.
Best thing they ever did is to go to those pressurized coolant tanks.