dB gain --> command-line volume switch?

Hiya.

This isn't really a MAD question, but it was suggested to me to try 
this list:  I'm trying to implement some support for the output of the 
'normalize' package in my mp3 player of choice, but ran into the 
problem that it doesn't seem that either mpg123 or mpg321 currently 
support the RVA2 volume adjustment tag in ID3v2.4 tags.  So I wrote a 
little routine in the jukebox wrapper program I'm using ('Tunez') to 
pull out normalize's backwards-compatible ID3v2.3 "XRVA" tag data.

Practical upshot is that before I call mpg321 I know that I want my 
file to be played, say, 5 dB louder or 10 dB quieter, but don't know 
how to translate that into the " -gain " argument's 0-100 scale on 
mpg123 or mpg321.  I could figure this out by trial and error, but was 
hoping there might be a more definitive conversion.  Anyone have any 
thoughts?

Thanks!
- Ert Dredge