Ripping audio - choosing mp3 bit rate

when ripping audio from a dolby 5:1 Dvd source, to stereo audio only e.g. for playback in car. -
typical media info in gspot or in power dvd would be 448kb.
this is spread across 6 channels so the main 2 stereo channels cannot be more that 224 each , and in fact must be less.

  • so what is the optimum mp3 bit rate setting in dvdfab ? - pointless to use say 320 when that is higher than source value & hence just a waste of space,
    so any formula for converting from gspot/powerdvd dvd info to dvdfab best settings ?

also, is there any info to be had on what mp3 encoder is used by dvdfab.
I have the free Lame encoder on my PC which is highly rated - can i configure that as a DVDfab plugin or is the DVDfab one just as good anyway ?