title: How to set a constraint using percentage?
body: <p>Just a very direct question, but we had spent many hours trying to find a working solution but faild.</p>
  
  <p>In Xcode, storyboard, how to set a constraint so one view can be located <strong>30% of total window</strong> height from the top of the superview? And we need it to be that way for ALL supported iOS devices of all orientations.</p>
  
  <p>Please see my illustration attached.<img src="https://i.stack.imgur.com/2rJm1.jpg" alt="enter image description here"></p>