title: Making a UIButton a % of the screen size
body: <p>I noticed certain button sizes look great on one the iPhone 5 simulator but do not look as good on the iPhone 6 simulator and this is because the heights or constraints that I place on the UIButtons end up leaving a lot of blank space down the bottom of my App Screen.</p>
  
  <p>I would like to have a button that is 40% of the screen size regardless of what device I am simulating on. </p>
  
  <p>Any ideas on how to make the size of the button stay 40% of the screen size regardless of the device?</p>