I don't think this would be necessary if folks ensured their X server's Screens' resolutions were configured correctly.

    $ xdpyinfo | grep -A 2 '^screen #'
    screen #0:
      dimensions:    1920x1080 pixels (483x272 millimeters)
      resolution:    101x101 dots per inch
    $

The all X clients would know how many pixels represented a physical measurement on the screen.