Here is what I do.
(You can play around with the below resolutions depending on what your are going for. I tend to make everything at 960x640. Just add 150 or so pixels to both)
1. Make all your layouts 1100x800 (or 800x1100).
2. Make your window size 960x640 (ipad resolution).
3. Create a sprite that is 960x640 and just make it a solid color. Set it's opactity to 50 or something. Put it at location 550x400.
This sprite is going to act as your guide for your window size. Anything that should be seen in the window should be within this guide.
4. On start of layout - Set ScrollX to 550, Set ScrollY to 400. (swap if doing portrait)
Now, your background image should be 1100x800. You want the image to be something that is OK if you don't see all of it around the edges. For example in a pool game. the background could be the concrete around the pool, if you see less or more concrete, it's no big deal. Or just use a solid color.
For fullscreen, if you are using CocoonJS, set to scale outer.
Other platforms you will need to play with. Windows 8 believe scale outer works, windows 8 phone, lettbox scale works.
Hope that helps. I have only used this on Static games screens (not games that scroll around a larger layout).