bitrate cal is included in xvid itself i think...
and u can get many of them from
www.doom9.org but u just cant get bitrate for ur need from any calc..
u need to test diff bitrates on a small part (say 5 or 10 MB) of ur movie...
mostly..select bitrate around 700 to 1000 in xvid if ur muvie is VCD/DAT/MPG (means non-DVD quality...) and mp3 96 kbps...
and if u have DVD files, then u can lower the bitrates of xvid...but u need to test it for quality on small parts...then comapring that u can judged it...and in future u can easily do the stuff...
so try to test for small part first...and if u like then only do for full muvie...ok?
for tutorials on xvid,VDUB,VDUBMod goto
www.doom9.org
all the codecs and tools are explained.