Anyone using x% for aspect control?

So my motivation is I recently bought a new laptop with 16:10 (1920x1200) aspect.  I've always designed for 16:9 (1920x1080) and set my addon.xml <res> to point to the single xml folder for any aspect.  So the UI is not filling the screen.  For backgrounds it is easy enough to switch

xml:
<width>1920</width>

<height>1080</height>

to

xml:
<width>100%</width>

<height>100%</height>

That fills up the screen, but now to adjust the controls.  Mainly things that should display at the bottom.  So I am trying to use a mix of <top>x%</top> and <bottom>x%</bottom> tags to position controls but it's not going well.  Partly I think there are a lot of nested controls and slide animations that complicate things.  But is there some sort of best practice if for example I have a control with height 75 that I want right at the bottom of the screen for both 16:9 and 16:10?

scott s.
.