Create 80/20 (or any custom) layout

[szantog]

How can I create a new layout 80/20 against 70/30?
I found two way, but these seem to be a little hacky:

1. I am able to edit the css, so I can hack the grid classes.
2. I can edit the screenlayout_2 template, and change grid classes to span9/span3

Is an easy way to define my layout?