I use a similar formula, but I convert the cubic inches to cubic feet diffrerently:

RPM divided by two, times cubic inches, divided by 1728, the number of cubic inches in a cubic foot. The result is theoretical CFM, with 100% volumetric efficiency, which never happens in the real world, especially on a street engine. Multiply the theoretical CFM times .85 or .9 to get a real-world estimate of actual CFM. Your 307 won't need over 600 CFM under any circumstances I can imagine, including wide-open throttle operation in first gear! (And if you do that very often, carry a basket to take the parts home!)
Jerry

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