There are many considerations when framing a photography and not really any specific formulae that can be applied that works in every case.
The best thing to have is a good framer who can work with you to select the options for each specific image. We use Mark Moore from Papakura Picture framers who is excellent (and patient) when helping sort through all the options.
But in a a response to a readers question, Mike Johnston posted these two articles with some more information.
Q&A: How Should You Frame a Photograph? Part I
Q&A: How Should You Frame a Photograph? Part II