My problem is almost the same, as described by Abioye Dez. 2013 under “Centering Megamenu”
TheTarget was to limit the width of the Megamenu to the same width as the Header.
My primary problem is displayed in first picture, where the width of Mainmenue and Footer is much bigger (marked in red) than the rest of the page . For the header and the rest it works fine (marked in green).
The suggested solution by Admin was to modify the templates/t3_bs3_blank/tpls/blocks/mainnav.php
in the following way: See Picture: Sugestion...

If, I follow the advice of Admin the Width is reduced but I get a double Mainmenu as you can see in the third picture.

Are there any ideas, how to solve this problem?

Thanks very much in advance.
Axel