responsive and whitespace

hi,
I am using a stretched layout and the site is responsive. max container width is 100%, content/sidebar ratio is 80/20, but I don’t use sidebars. Content + sidebar width is 70%.

When I open my site on a screen with 1900 screen width, everything is as it should be. The fullwidth slider, the small bar above the main menu, the socket and the footer are shown correctly with maximum width. The main content area is centered with 70% width.

My problem now:

If I reduce the width of the browser window/tab I don’t want the main content area to start shrinking immediately, but instead stay as long as there is whitespace on the right and left side.

I want the main content area to start being responsive only when there is like 20px or 1cm whitespace left on both sides.

I hope you do understand what I want ;-)

Thank you,

rob