Currently the DPI setting in your OS has no relevance to the actual DPI setting being displayed.
Uhm, that was my entire point. Hence my reference to typesetter's points, which are strictly a unit of measure. (Or were, until computers got hold of them. "12 point" fonts on that old 72 dpi monitor ranged from 12 to 15 pixels. I think New York may even have been 16.)