The carbs catalog on the Holley.com, pages 24 and 25 (pdf pages 18 & 19) has a basic guideline for carb selection, that will do just fine in most cases. I suppose they know what they are talking about. The catalog has all sorts of basic info on carburetted systems, very informative.
http://www.holley.com/data/Catalogs/...arburetion.pdf