need to make horizontal spry menu fill div width

I am trying to make a horizontal spry menu (CS4) fill the entire width of a div centered on a page. Through various changes to the "ul.MenuBarHorizontal a" style (padding, text size ect...) I have been unable to make it fit the exact width of the div. I do not want fixed width links as the character count within the links varies too much for this. Any standard way to do this? An alternative would be to fill in the end of the menu bar with a "blank" which I have tried but because the graphic buttons I have placed in the menu links are, I guess, proportioned to fill the auto height of the "ul.MenuBarHorizontal a" and "ul.MenuBarHorizontal a:hover" styles, the "blank" I insert into the "ul.MenuBarHorizontal" style background is not proportioned the same and therefore looks different. Anyway to get it to proportion the same as the "a" and "a:hover" images. Sorry about the long-winded question, either making the menu fill the width, or making a "blank" would suffice. Thanks in advance.