I remember an equation that gave you a ball park CFM based on cubic inches and maximum RPM.
(Cubic inches X max. RPM)/ 3456
Take your max RPM the engine is expected to see, times the cubic inches and then divide that number by 3456.
So, if you have a 351 and you want to see 6000 RPM;
351 X 6000 = 2106000, Then 2106000/ 3456 = 609.375.
Round to the next hishest available CFM = 650 CFM. I imagine if you shop around you could find a 625.
If you go lower, you will bottom out on available mixture transfer.
Just my $0.02
