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.