I posted this in the Doom 3 forum but I might as well post it here too. This was done for The Dark Mod which uses the Doom3 engine so it should be relevant here too.
Over the last few days I've been working on a guide on how to split patches, it's almost complete and is up on the DarkMod wiki.
You can find it here: Here
EXAMPLE: http://img708.imageshack.us/img708/7826 ... atches.png
EDIT: Sorry, I posted this from an iPhone, fucking POS interface.