yes thats the right syntax, ive tested it here and it works fine
example:
you have a window with width 100 and height 200, and an element that is 20 dlg units away from all sides
code:
<Window Id="TestWnd" Version="1">
<WindowTmpl />
<Position Height="200" Width="100">
<Resizeable Allowed="BothSides">
<MinWidth>100</MinWidth>
<MinHeight>200</MinHeight>
</Resizeable>
</Position>
<Elements>
<Element xsi:type="FigureElement" Id="Background">
<Position Top="20" Left="20" Width="60" Height="160">
<Anchor Horizontal="LeftRightFixed" Vertical="TopBottomFixed" />
</Position>
<Figure><Rectangle /></Figure>
<Fill><PlainColor><BaseColor/></PlainColor></Fill>
</Element>
</Elements>
</Window>
when this window is resized it will always be 20 dlg units away from all sides.