I always position all of my monitors at the same distance from my eyes. That's because I'm an old guy and my eyes can't focus to different distances easily like they used to. So I have a pair of single vision computer glasses - like a reading prescription, but adjusted for the approximately 20" between my eyes and the laptop screen (almost exactly the half-meter you mentioned).

Typically I have three displays in a row: a monitor on the left and two computers on the right, a MacBook Pro Retina and a ThinkPad W520 (145 ppi). Whichever computer I'm primarily working on that day goes in the middle and is connected to the external display.

I wouldn't be able to use a display 34" from my eyes unless I used a different pair of glasses, and then I'd have to switch glasses back and forth to look at the different screens! Unless I gave up using the laptop screens at all, but I'm not about to do that.

So with all my monitors at the same distance, what feels like a "retina" density is the same on all of them.

Beg your pardon, I wasn't trying to suggest that you should sit further from the screen, not at all.

I was just stressing the fact that depending on the size and distance, even sub-100ppi screen densities might be good enough.

Admittedly if you sit 20" from the screen regardless of its size you may want a slightly higher ppi.

I wasn't disputing what you were saying - just pointing out that the idea of a lower pixel density on a monitor that's farther way doesn't work for me, because my eyes can't focus back and forth like that. All my monitors have to be at about the same distance.

If someone can easily focus at different distances, then of course that opens up more options.