As far as I know there's not an option of 'center' the submenu under the main menu item that calls it. But, there was a discussion about a function to find the exact position of a horizontal menu item so that one could then set the drop down submenu exactly as one wanted. http://milonic.com/forum/viewtopic. ... light=gpos Hope this helps. I believe they actually figured out a function to do that.
Looked at the topic You mentioned. It was just a little bit more avanced solution, than I was hoping for. So I just hardcoded the subitem width and offset.
Yes, MRI it is, I'm still trying to figure it out because I want to center my submenus but also leave my main menu in the screen position center. So,that precludes using the left offset since the main will change position depending on the what is center in a particular user's browser.