Well, I realized since I posted this that I was I incorrectly reading my manometer.

The correct column height was actually 4.8" which represents an unrestricted flow of 280 CFM.

With that much flow this will be just as accurate as a professional apparatus would and will give the same kind of comparable readings (ie CFM @ X inches of H2O).

I am not doing this to be able to compare before and after numbers though. This is for the purpose of making sure that all the head and manifold ports have the same numbers so that my air fuel mixture is the same in all cylinders.

Between this and matching chamber volumes I am currently obsessing on perfect volumetric symmetry.