I'd like to bump this old thread, as well. The expression is incredibly useful, but I can't get it to keep my sun within the layout.
My layout is 1600 by 900. I want the sun completely within that layout throughout the day, but whenever I change the X expression, it ends up being left-aligned rather than centred. Any of you guys have any ideas?
Here's the layout with the basic elements: [attachment=0:iaw3grlp][/attachment:iaw3grlp]