I have only used the Holley formula above to get a ballpark size. But most rookies think their engines will attain 100% VE. Most will NOT. Not even close to 100%. I think Smokie said the average VE most street engines attain is around 85% with basic "Hot Rodding" of a hotter cam and headers. YMMV!

My 410CI stroker 5.9 with full ported Eldy heads, warm cam, 10.25:1 CR, shorty 1.75" headers may be around 90% VE and at 6500 RPM MAX says I'll need a 694 CFM. I will be running my 850 CFM ThermoBOG on an Air Gap manifold.