For a Carb the formula is, (cfm)
((rpm x displacement)/3456)x1.1
e.g. Chevy 350 (built for 7000rpm)
((7000 x 350)/3456)x1.1 780cfm
A 650cfm carb is used. (Big engine and RPM!! so dont worry about big figure
)
Thats just an example, not sure whether there is one for TB`s.
Going for the size smaller but closet is better to apparently.
(from Auto Math Book)