This is what 35em looks like on my computer. My laptop has 1920x1080 resolution but only a 15" screen, so I have to use 125% zoom on Wikipedia to make pages confortable to read. The result is a lot of wasted space where a second column could fit perfectly:
My comment on the original patchset pointed out that you shouldn't guess this kind of thing and should be able to point at why using the fact that there was a difference between mobile and desktop as an example I'm not a designer so I don't know what the right value is. I suggest you get input from a designer rather than guess.