I recommend used aspect-ratio (serach wiki), adjust  mockups a initial ratio , and generate all mockups from a list aspects-ratios like [apple IOS aspect-ratio][2], and for [android aspect-radio][3] use actual sizes "like" as : 2:* "small" , 4:* "normal",  7:* "large" ,10:* "xlarge"


 
  [2]: http://www.iosres.com/
  [3]: http://developer.android.com/guide/practices/screens_support.html