Drop Down Menu: Horizontal Positioning

Hi all,

I have a problem with positioning my dropdown menu.

Can I position it by referencing the button image that triggers it? Or atleast to a specific point.

The main trouble is horizontal positioning. The main part of the site is 760px wide and positioned in the middle of the page. Obviously my trigger button is going to be in a different position on different users screens.

Could I position the menu div in the middle like my main site then use left margin to align it from there?

PLEASE HELP!!