Fix nav menu to width of content

Hi There,

I am looking to fix the nav menu so it fits to the width of the rest of this site the container is a fixed width of 940px along with the rest of the site but I was wondering how I get the menu items to stretch to the 940px width. I’ve tried to do this using padding, but its not very accuract and I was wondering how this might be done automatically.

Any help would be great thanks