Attached is a worksheet that might help you when you’re brainstorming the design of your Windows 8 app.
It encourages the 3-tier navigation structure that’s common with these apps. The grid units are 1 unit (20 pixels). There are rules for the top and left margins (5 units and 6 units respectively) as well as for an app bar (88 pixels). The Windows 8 version includes a guide for snapped view (320 pixels), whereas the Windows 8.1 version has minimum width guides at 320 and 500 pixels.
If I modify it, I will replace the attachment here so that you can always come back to this post to download more sheets. It helps to have a few of these printed out and stashed in your computer bag.
Hope it helps.
Windows 8: Windows 8 App Design Sheet.pdf
Windows 8.1: Windows 8.1 App Design Sheet.pdf