Set reference point for multiple objects in the middle of the document

Hello Community,

I want to mirror a layout.

So I:

1. Select all the objekts

2. Set the reference point to the middle

3. Mirror the objects

BUT

The reference point is the middle of all the OBJECTS. That leads to several objects crossing the border of my document because the layout isn't spread out evenly.

I want to middle of the DOCUMENT to be the reference point. How can I do that?

Thanks

Christian