If I remember correctly, by defining the viewBox, you set the coordinates for your page, that is, you can then put something in the middle by specifying (300,400) as the coordinates.
As for the ratio and preferences, see the
preserveaspectratio property. With "XMidYMid meet" you set it to take the most space possible while preserving the aspect ratio of the page (it's desirable for images, it may not be needed for a title page like this), and, if the actual screen is larger in some dimension, the page is centered.