You can calibrate Zoom 1To1 (Zoom1To1Calibrate command) more accurately by using the dpi (actually, pixels per inch) or the dot pitch of your particular monitor than by measuring the calibration bar with a ruler on the screen.
A dot pitch and dpi calculator is available here.
There are also tables of known monitor resolutions available, for example here.
Example: My laptop screen is 1920 by 1080 and 15.6 inches on the diagonal. The dpi is shown as 141.21. The calibration bar displayed by Zoom1To1Calibrate is 478 pixels long, so 478 pixels/141.21 pixels per inch = 3.385 inches, is what I enter as the bar length.
To set the length of the calibration bar in millimeters, use the dot pitch number, rather than dpi, in my case .1799mm. So - (478 pixels) X (.1799 mm per pixel) = 85.99 millimeters as the bar length.
Keywords: Zoom1To1Calibrate; Full Scale