Windows 8 Modern UI Apps - How to Snap at center

There's no way to do this. You can only snap an app to either side of the screen and make it 320px wide or the remaining width of the screen. You cannot make them 50% the width, it's not possible.

Microsoft has a good reason for this: so that app developer can focus on what to show when you snap the app to the side. Remember that this is not the same as snapping in the desktop environment, when you snap a Modern UI app to the side, its content has to change as well to match that position. Allowing freedom to choose percentage of the screen to snap the app would make it harder for app developer to code their content accordingly