Center fpsnode on screen

i've enhanced the fpsnode from one of the tutorials. it can now display multiple lines (simply creating several text nodes and modifying their y position) and supports appending text (pretty much like a jtextarea). however, i don't quite understand why the text starts at the lower left, why it isn't "frustumed away" (it's at 0/0 x/z)), but i'm not really interested in that. i simply want to know how i can center it. i gave it a bounding volume, but its worldbound still is null, so i don't know the textarea's size.

bottom left is zero

look at JMEDesktop and how it gets centered, should be the same screenWidth / 2 - myWidgetLenght / 2