Measure Screen DPI
This simple script allows you to fine-tune your X server and ensure that
screen distance as set by Tk_GetMMFromObj or get by Tk_GetScreenMM are
in actual millimeters (or inches, if you prefer imperial units).
Script draw grid 10x10 cm on the screen. You measure it using ruler and
enter measured value into entry below the grid. Then press a button and
script calculates your real dots per inch value. You can than write it
into x-server command line, and X would report real screen millimeters
to programs.