All of the calculators do the same thing: allow you to not use your computer's calculator to do this:
CFM = displacement × volumetric efficiency × peak power RPM ÷ 3,456

What it doesn't do:
1. allow for any manifold design except single plenum (the results for IR, divided single plane and dual plane will be completely wrong)
2. factor in manifold plenum volume (treats OEM low rise 273 manifold exactly the same as shoe-box tunnel ram)
3. factor in how nominal carburetor flow (what the manufacturer says) changes when it's not exactly the "right" size.

An 850 on that engine will flow more than 850.
An 850 on a 273 will flow less than 850