From The Document Foundation Wiki
How can I change the zero point for the vertical and horizontal rulers?
Follow these steps:
- Click the button at the intersection of the horizontal ruler and the vertical ruler in the top left corner of the workspace.
- Drag the cursor to the point you want to use as the new zero point. The point's current location will be indicated by crossed dotted lines.
- Release the mouse button at the desired location.
To reset the zero back to its default position, click once on the point in the top left corner where the two rulers intersect.