edelbrock use to publish a formula in their catalog some years back. they said do the cfm calculation and then multiply by 110% for a single plane and 125% for a dual plane. they no longer publish this and i'll bet it's because their carbs, multi-carb packages may not fit into their formula. anyhow, I think the edelbrock formula is better than the holley for the average guy doing a single 4bbl set-up. as for me I've never gotten out of the venturi area thing and only treat cfm ratings as a manipulated tool.