I did use the square of the diameters in my Estimate. I also took into consideration the effect of the transition of the charge from 12G to 10G as it past through void created by the 10G forcing cone. After making a scale drawing of the barrel with the gaugemate in position it appeared logical to use the square of .752 diameter in my pressure calculations[ie;.752 diameter being the mean diameter between 10G and 12G].Experience has shown that when faced with this type of problem it is adviseable to go back to first principals, in this case Boyles law.I would agree the proof of the pudding will be apparent once controlled tests results have been evaluated.
By the way as a boy, I used a Webley and Scott .410 adaptor in a 12G; it was effective on bolting rabbits so balistics was not an issue!


Roy Hebbes