I don't think it is possible to give a 'correct' answer, as such thing depends on many variables. For example, do users enter the site in explorer mode, or in task-completion mode?
Anyhow, analytics data such as Average Time on Page (ATOP)can give some guidance for this ratio - the larger the ATOP is, the larger should be the content area.
The existence bias
What I do feel important to mention, is that the question has a strong scent of existence bias - The attribution of goodness to things merely due to their existence. Specifically, the fact that you assume that navigation should take space!
This is obvious as such is the convention on many many pages, but one has to consider that this pattern originated well before the web was what it is today, at times where many of the current technologies were yet to exist.
I think one of the great things that happened in UX is the boom of mobile interfaces. The fact that screen real-estate is scarce forces you to think differently, think small, and seriously consider what goes where. Personally, I find that mobile-first approach to UX is beneficial, even if I design for desktop only - it is much easier to throw a few well-defined small black-boxes into a big desktop page than turn a big desktop page into well-defined small black-boxes.
Also, the popular-on-mobile hub-and-spokes navigation pattern is far more 'coherent' than the complex network of connections created by the introduction of global navigation.
Anyhow, with relation to your question, consider the following screenshot of a BBC site on Chrome for iPhone:
First, notice that Chrome utilised a pit-bar, which is unseen in this screenshot due to scroll-down. The pit-bar, where the Chrome menu is, appears 'on demand'.
Also, note the 'Menu' and 'All Sport' menu triggers (and there's even one for 'Football'). Here as well, the navigation will show 'on demand'.
These are just three of examples of many on-demand navigation elements that appear on mobile devices.
And so the question is: why do navigation elements on desktop are always visible rather than be on-demand?
Obviously, the answer for this would be to save the user some physical effort (scrolling or clicking), but is it really worth it on content-rich sites, especially when modern web technologies offer very elegant solutions?
My point is, that by adopting modern technologies, a bit of creative thinking (or phrased alternatively - not accepting historical conventions to be right by default), a better question may be - how do I show navigation on-demand?