?
first i choose an empty DinA4 page.
i take two guides from the top and the left to cross them in the middle of the page.
than move the ruler zero point to this crossing of the guides. and it snap to them.

if i zoom very close i can see that this zero point does not fit to the magnetic guides.
you can even choose the properties of each guid to see this.
And its getting worse if you choose a scaling factor (i have to) 1:20... .

So how to snap the zero point to guides and objects EXACTLY ???