Using the file-new-label option I created a custom sized label. I dived an A4 in exactly 16labels without spacing between them. So, I selected a horizontal distance of 5.25cm and a vertical distance of 7.43cm. The width and height were exactly the same and I the top and left margin were 0.00cm. I used 4 columns and 4 rows and a paper size of 21.0cm by 29.70cm. 

When printing this on a pre-printed A4 sheet with no margins I found that the text of the bottom row was shifted on the label with respect to the pre-print in comparison to the top row. I checked what happened by measuring the distance between the text and found out that the actual printed distance was exactly 1mm larger than specified. So the horizonal distance turned out to be 52.35cm and the vertical distance 74.53cm. 

As a work-around I created a label sheet where I used distances, widths and heights that were exactly 1mm smaller than it should be and now the print is perfect. Downside is that on the screen it now appears as if I am not using the entire A4, it shows like there is a blank column on the right of the labels and a blank row on the bottom. It would be very useful if the user can actually use the correct dimensions and that it gets printed accordingly.