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) and for snapped view (16 units).
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.