Trial & Error is the way to do it with digital. I have watched like 2 hours worth of video from Nikon about their so great TTL CLS system for multiple flash unit. And I learn that it's all Trial & Error. So with film just have everything in manual and use a flash meter like others have recommended. Using TTL for multiple flash setup I don't think it would work as I have not seen or know about someone who can set up such a setup and got the right result the first shot. It's always trial and error and if it is then the Auto TTL is basically just get in the way.