I created a tool that allows you to scale/adjust your layouts for tablets and small screen devices and made a blog post about it here: http://onemanmobile.blogspot.com/2012/04/how-to-scale-your-android-layouts-to.html
Basically, defining your layouts in dp units is not enough if you want your app to fit on all devices and tablets, since there's four different "density-buckets". This tool will allow your layouts to be converted into fitting these density buckets.
So in your case, define one layout - and have it scaled as you please..