Full width Sub-menu

I have a standard menu with dropdown items. I want to edit the sub-menu and make it span over the whole width of the site (1250px wide & horizontally centered). I was able to make the sub-menu UL 1250px wide, however I cannot make it centered or absolute.

#nav .sfhover ul,
#nav :hover ul {
	position: absolute;
	display: inline-block;
	top: 100%;
	width: 1128px;
}

On-top of the WP “default.css” is the above css I added to my custom “style.css” file. Am I even calling the sub-menu right? I tried using “.sub-menu” AND “ul ul” also with no luck.