I never liked the scaling options used in Ubuntu for HiDPI screens.
I tend to leave it to 100% and adapt the fonts and let legacy applications adapt scaling themselves (X-Windows).
On the fonts side, I force DPI to 144 and set font size to 12 for my 32" 4K screen.
And I tell the browsers to adapt their own scaling - which provides a better (sharp) result for me.
I know it is not optimal, but that works fine for me on my calibrated 4K screen (External screen).
I use the DPI / PPI calculator to get the main details because the PPI changes with the physical size and the resolution of the screen.