The formula does something useful - for a single plane manifold with minimal plenum volume.
Any other type (dual plane, divided single plane, tunnel ram, individual throat) is different. Your iron manifold and the Edelbrock RPM are both dual plane, and the carburetor predicted by the formula is too small.
Regardless of what Holley says, a 750 CFM carburetor will flow 750 if the engine draws 1.5 Hg" @ WOT. E.g., a 440" engine @ 6,000 RPM and 100% efficiency calculates to 764 CFM, so that carburetor is a good (but not only, best, or fastest) choice.
If the engine is smaller with lower WOT vacuum, that same carburetor will flow less.
If the engine is bigger with higher WOT vacuum, it will flow more.