The header tanks primary role is to provide room for expansion. So as long as there is "some" water in it when cold, that should be all it needs. But it can be good to have an excess of water purely as it increases the volume of coolant to the entire system.

"Some" is only really required so you can actually see the actual cooling system isnt dry. And in some ways, the more air space, ie expansion room the better. But that's the main role of the bottle. Too little and it can get blown out.

Too much....cant cause any problems.