The automatic calibration was like 10% smaller than the real size and I used the credit card method [1] It need to show the "crop lines, no bleed" lines to make the comparison easier https://toastdesignservices.co.uk/resources/what-crop-marks-...

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