but my monitor does not have the real DPI the label on it's back says... configuring by DPI is moot.
i read a little more on the link you provided... and the closest to the gimp old way of doing that is putting the width and height of the viable area of the monitor in mm. think that might work rather well if it's correctly implemented.
I wasn't suggesting reading a label. One measures the visible picture in real life, e.g. ruler, and calculates the DPI. CRTs didn't have a label on the back and, anyway, one could adjust the picture size.