Looking at the wps file, the relevant line is the following:
" %fbk"
(quotes added by me)
So the bitrate is always indented by 8 spaces. You can either change that number of spaces, or if you (for example) want it centre-aligned, change the line to:
"%ac%fbk"
For more information, see
http://www.rockbox.org/twiki/bin/view/Main/CustomWPSEDIT: I've just looked at the actual WPS (in the UI simulator), and see that the bitrate is displayed in a box offset a little from the left side of the screen (so the %ac tag isn't what you want). If you want to centre-align the bitrate within this box, then you'll need to use a "viewport". The CustomWPS page describes them.