Hi Hidekazu. the 320px / 20px gap mentioned in the Windows Design docs is "effective resolution" (aka "view resolution"). All the pixel values in XAML and HTML are in effective resolution :). Sorry, I don't have any expertise on desktop apps; Steve Wright had a talk on desktop apps and high-DPI support (http://channel9.msdn.com/Events/Build/2014/2-535)
Please note that the slides available on this page are NOT the ones I presented; please see the slides from the repeat talk to get the actual slides (but sadly several of my demos failed during that talk, so you should watch the video on this page!)
For some reason the slides are stuck - they seem to start working at 25 minutes in. I'll see if we can get this fixed but I doubt it. You'll just have to load the slides and manually follow along I'm afraid . Sorry for the inconvenience.