How Do I Make The Inner Primary Navigation A Fixed Width

Hi,
I'd like the inner container of primary navigation a fixed width so it displays with the same width on all desktop screen resolutions.

I know I can go to "layout" and set the inner navigation to "contained". However, I would like control over the actual width it is contained in. I know there is a way to change the padding of the navigation so that it is contained either side by a fixed amount, however this will change the width of the menu on different screen resolutions.

I set my navigation as "Use Navigation As header".

I also used the elements module to merge it with my content.

Thank you in advance.
Marc