1) Plexi glass scratches easily - even wiping dust off over time will dull it up. I'd use tempered glass.
2) The screen angel - Idealy you want to me looking at the center of the screen, with your eyes parallel to the screen angel, so its totally natural to look at. But this would require you to mount your screen around the same hieght as your head - so its not very practical.
I mounted mine so when I look done at it, the screen is angle is titled to match my viewing angle. Which means for my cabinet, its angle'd up a little bit. I'm currently building a driving cab, where the screen will be mounted slightly above my natural eye position, so it'll will be angled slightly down to match.
I hope that helps a little. Its really trial and error for what works for you.