As for me: I haven't figured it out yet, sometimes a fractional scale works, sometimes you get vague errors. In any case what I have noticed is that quality drops significantly if you use fractionals.

For my set-up at home (1080 - 4k - 1080) I have a simple script, that really was more of a `write once - use always`-experience. E.g.

    xrandr --output HDMI1 --auto --scale 2x2 --pos 0x0+0+0
    xrandr --output eDP1 --auto --scale 2x2 --pos 7680x0+0+0
    xrandr --output DP1 --auto --mode 3840x2160 --scale 1x1 --pos 3840x0+0+0 --primary

So basically what I do is, I virtually up-scale my other screens up to 4k, such that system-wide zoom settings are (more or less) consistent and enjoyable.

No, with `xrandr` you can scale to any fractional amount. For example, I had 2 24" external monitors (full-hd, not 4k), and a 13" thinkpad (chromebook, booted into linux). Obviously, the respective DPI's were way off.

Just a quick calculation in a repl, then called xrandr with the right scaling number, and presto, I had them exactly the same.

Then, with precise lining up of the displays, it was basically one seamless "sheet of paper". If I had a long web page or pdf spanning the displays (the laptop was directly under the monitors), then scrolling through it was quite "trippy".

I think because this was now a unified display where the bottom 1/3 was tilted differently to the top 2/3'rds (think of a curved ultrawide, but in portrait mode)

Honestly, it really was that simple.

If you want fractional scaling I’d suggest you look into wayland.