This app is created to facilitate the calculations of percentage via Percent Support Library
Here you have an random design with screen size 1920x1080 :
And we see that the height of button is 230px. So the screen height is 1920px, and heightPercent for this will be 12 as you can see on calculator:
And finally XML looks like:
To integrate Percent Support Library, add in gradle: dependencies { compile 'com.android.support:percent:23.3.0' }