I copied this from Holley:
How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency
(Example: 350 CID x 6000 RPM = 2,100,000 / 3456 = 608 CFM)
Approximately 608 CFM would be required for this engine. However, most Street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.
Next, you need to decide whether a vacuum secondary or a mechanical secondary carburetor will work best for you.
As a rule of thumb, vacuum secondary carburetors work best on:
Relatively heavy vehicles
Engines built more for low-end torque
Conversely, mechanical secondary carburetors seem to work best on:
Relatively light vehicles
Strip gearing (4.11 or numerically higher)
Engines built more for top-end horsepower
Hope this helps