This is exactly one of the types of things that Auto Layout was meant to solve. Behind the scences, everything is using Auto Layout now, but if you have a lot of hardcoded constant values for sizes, you're in for a lot of work to actually take advantage of its new features.
If you want to avoid storyboards/xibs, I suggest considering refactoring your code not to use static sizes via frames/bounds (i.e. a lot of initWithFrame calls), but rather use Visual Format Language where ever you can, and dropping down to full on NSLayoutConstraints as needed.
You'll want to read and understand this: Working with Auto Layout Programmatically
With VFL, you can express virtually all of your sizes and positions as relative values. Where you cannot, you can pass in metrics to your VFL calls with the values that need to be explicitly set for iPad v. iPhone (where relative, calculated values do not work).