I can use 2K 13 at 1x quite well. It’s the “goldilock” for me.
I think that is around 170 ppi if not wrong. Im boarderline comfy with that.
I set my 3K touch to 1600x900 which is a 141 ppi.
It also depend on the quality of the backlight. If the panel is very constant and not flashing I can keep it more dim and closer to me than a bad panel.
The 15 inch I currently had a measly 1366x768 and thus an “atrocious” 107mm ppi. Things appear gargantuan, thus I can keep the screen much much further away.
That’s the problem with using this 15 inch – everything else feels small. You can’t get a comfy posture with an external keyboard. Although I would be then complaining about my chair.
But I used to run 2K 13 inch and is perfectly fine.
2256x1504 have a whoppin 208 ppi, which make sense. You basically need magnifying glasses.
I would say around 150 ppi would be considered a good balance between real estate and eye strain.