Doesn't this work? Add this in the end of your CSS.
#nav ul.subs {width: 100%;}
It worked for me! :)
Or if I understood wrong, you want the mega menu to start from the menu's left position. If that's the case, you need to position
the menu links as relative
and the mega menu as absolute
.
The solution for this is:
#nav > li {position: relative;}
#nav ul.subs {position: absolute; width: 500px;}
I guess you can take it forward from here: