(Admittedly, "points" are still likely a good measurement for print. Perhaps one can work backwards and fudge point as a measure of angle if you consider 12 point font at a typical viewing distance.)
I assume the real hard piece is figuring out the distance the display is going to be viewed at. Some definite defaults exist (phones are typically about the same distance away, same with desktop monitors) but unique situations certainly can exist. (I'm also assuming that the monitor can report it's physical size and resolution; combined w/ viewing distance, it should be possible to calculate FoV.) If you did this, you should be able to mostly seemlessly split a window between two displays, and have it be equal "size" in the FoV. (of course, some displays have borders, so that fudges it a bit.)
That is a good starting point for calculating the default "optimal UI scaling", but there are going to be adjustments needed for the FoV of the whole screen area (not per pixel) too.
With large screens, for example 24-30" on your desk, just the per-pixel FoV measure will probably be good enough. You have plenty of "space" for windows and content, and want to get the optimal scaling.
But once you get to very small screens like phones, there is a tradeoff between keeping font and UI sizes comfortable, and being able to actually fit enough content on the screen without endless scrolling. I am willing to strain my eyes with smaller font sizes on my phone than on my laptop, just so that I can see more than 5 sentences of text at the same time.