If you’ve ever written code like this:

  CGSize size = self.bounds.size;
  CGRect r = CGRectMake(MARGIN, MARGIN, size.width - 2*MARGIN, size.height - 2*MARGIN);

Do this instead:

  CGRect r = CGRectInset(self.bounds, MARGIN, MARGIN);

And while you’re at it, go read about CGGeometry