Eh, I suppose. Just the criteria of "is it hidpi? yes/no" readily mislead GP for example (i.e. it definitely is, just still "not hidpi enough"), so I felt it would be helpful if the mechanism at play was clarified. Maybe I came off too strong though. Felt it would be clearer to use the correct variable at least, than to try and relativize PPI.
I guess, but even without measuring pixel inches/degrees it feels clearly wrong to me to say that proper 1x on a 12 inch laptop screen is only 960x600. 1280x720 or 1280x800 makes more sense to me, and then there's no confusion because 1920 is a clear 1.5x resolution.
For what it's worth, it's a pretty small diagonal size. Netbooks used to be about this size, and those had exactly such low resolutions on them. Conversely, you'd see 1280x720, but especially 1366x786, more on regular variety laptops (~15"), and if you crunch the numbers for these (using standard ppi), it maps pretty much exactly right. So we've come a long way on Windows/Linux/BSD land, even if there's much more to go.
3840x2160@15.3" for example would be a nice even 3.0x display scale, at 287.96 PPI, and 128 PPD at 30° hfov to match the line pair resolving capability of the human eye [0] rather than the light dot resolving of 60 PPD, although of course still far from the 10x improvement over it via hyperacuity that you linked to earlier.
I accuse those 15 inch laptops of being below the bar. 15 inch should be 1600x900.
If 960xwhatever is okay at 12 inches, then 1366x768 wouldn't even be the baseline resolution for 15 inch laptops, it would be the baseline resolution for 17 inch laptops. That just sounds silly to me.
Assuming the laptop screen is just 20% closer goes a long way here to figuring out a good resolution. And it gives 720p to 12/13 inch laptops at 1x.
It's wrong but it's wrong in a way that causes minimal trouble and there's no better option. And if you add viewing distance explicitly, PPI+distance isn't meaningfully worse than PPD+distance, and people will understand PPI+distance better.