I used something like this before to adjust the zoom in Word or Google Docs with the real size on screen. I prefer to overlap a A4 sheet on the screen. It's semitransparent, so it's easier to see the calibration. Also I'd like to use a transparent rules on the screen and overlap it over the virtual rule for calibration.
It's really clever, but the screen detection is overconfident; it gave me the green "calibrated" box while guessing a completely wrong display size for my laptop.
I used something like this before to adjust the zoom in Word or Google Docs with the real size on screen. I prefer to overlap a A4 sheet on the screen. It's semitransparent, so it's easier to see the calibration. Also I'd like to use a transparent rules on the screen and overlap it over the virtual rule for calibration.
[1] For some stupid reason, it sound scary