How do I calibrate the screen for 1:1 display?

Open the GP Admin module

and click on "Calibrate screen"

A window will now open in which you must select whether you want to align the square shown there to 10x10cm, 20x20cm or 30x30cm. Once you have decided on a size, drag the window in length and width so that the square (circled in blue) has these dimensions. Use a ruler, tape measure or folding rule held up to the screen. When the dimensions match, click on the [Apply calibration] button.

Close the GP Manager and restart it. Only then will the calibration be accepted.