Tim,
Do yourself a favor and get this, 'Finely Focused: Mechanics and Creativity in Large Format Photography':
http://www.circleofthesunproductions...elyFocused.htm
Have you tested your true film speed and development time as David suggested? If you shoot one film it may be worth some cash to have The View Camera Store do the film speed test and development time for you. They send you film, 5 sheets I think, with a step wedge exposed under controlled conditions on it, you process each sheet for different times send it back to them and they put it on a densitometer, run a program and out pops your film speed and development time (N) and N-1, N-2, N+1 and N+2. But it costs about $50.
http://www.viewcamerastore.com/produ...products_id=82
Do one or the other or both-it's up to you.
Good luck,
Scott