Containers Width

How to use percentage for widths?
I posted some questions before and the dev said to use percentage in sizing the containers but how?

I tried this and it is working but I want the width to be in percent so I will have a responsive left container when user resize the browser.

pattern: '3J',
cells: [
  {id:'a', text: 'Menu'},
  {id:'a', text: 'Data Chart'},
  {id: 'b', text: 'Data View, 800'}

How to make the 3rd container always get the 80% of the parents width?
I tried the following:

width: "90%"   // not working

width: 90%   // obviously this will not work.

How to use percentage? I cannot find it in the documentation “.chm”

I had to make a manual calculation to make this happen.