Max sidebar width only 8 units

I'm trying to build a new website, where the content on the front page is only 3 units wide on the right hand side, as it will be used as a news column.

I have setup both sidebars first. And my first sidebar block is 4 units, as it contains a menu.

The problem is that I need sidebar two to be 9 block units. I know the CSS is in place to do this, but I am not able to set the fusion layout settings to more than 8 units per sidebar.

Is there a way round this? Im using the fusion based Acquia Prosper by the way.

Thanks.