I read the article in the link that Tom Cimino posted. I have been adjusting centrifical and vacuum advances since the late 60s and have read lots of articles on the subject over the years. That article is the very best on the subject I have ever read, and covers additional issues such as compression ratio, and cam timing and how they effect ignition timing. If you are not an expert on the subject it is a great learning tool and it explains it all in detail. I would suggest everyone print all 12 pages of that article and keep it for reference. Thanks Tom.
Wayne