GUILayout.BeginArea expand to screen height?

I know you can use ExpandHeight to expand buttons/etc to whatever the height of a BeginArea is, but how can I make a BeginArea expand to the screen height?

So, if the resolution is 620x320, and the Y position of the BeginArea is Screen.height/3, how do I get the height of it to expand all the way down, and adjust when the Y position/resolution changes?