I'm not entirely sure about all of this, but I'm pretty close, so here goes nothing.
To center the bar, you want the X to be your screen width, minus the bar width, divided by 2, which is 330. And for the Y, you want your screen height minus the bar height, which is 914. So:
Code:
<Element ID="ToolbarField" X="330" Y="914" Width="620" Height="110" Detach="1">