I don't really know where I got the percentage thing, I think I just devised it working from stated as well as measured diameters of chokes in various gauges.
It could be a bit easier to say % of choke area in relation to bore area. Let B = bore diameter, C = choke diameter & R = % of reduction. The formula then is
C = √(B˛ x R) Thus for the 12 gauge √ of (.729˛ x 90%) = .692 or .37" of choke constriction.
For the .410 bore √ (.410˛ x 90%) = .389" or .21" of choke constriction.
These would be essentially full chokes.
