I'm currently working on a small game for iOS Devices and to get all ratios in place is a real pain :/
As far as I've tried, the best is to take 4:3 (all iPads) as standard ratio and expand to 3:2 (iPhone <4s) and 71:40 (iPhone >5).
And it'll look like this if scaled:
But actually it would be much better if it scales like this:
is there a possibility to adjust this?