Tab spacing moves back two mm

I am trying to set my indent on .50cm using the mouse to slide the rectangular indent icon (or the one above the triangular icon) along to .50
However, it will not stay on .50 when I release the mouse button, but moves back to .48cm. I workaround by dragging it to .52cm and it comes back to .50cm but it would be great if you could fix to make it stop at .50
It happens with both of the indent icons.

You do have to fiddle with the ruler a little - unfortunately, though, this is out of our control because the ruler is a standard OS X control (the same as the one used in TextEdit), and there’s no way for us to customise it’s behaviour.

All the best,