Pattern is the effect of the restriction that the choke causes when the shot to pass through. The bore is relevant only in that the difference between the bore and the choke area cause the pattern to be altered from true cylinder to a tighter pattern (form). Measure the bore and then the choke area. Write it down. Then shoot the gun at a patterning plate.
Your choke gauge can not work as a true indicator of expected pattern. The only thing you can learn is if one choke bore area is tighter than the other. Paper and counting is the only real test.
I have seen guns shoot patterns which were vastly different from what was expected. So many variables can change the pattern. I had a set of skeet tubes which were choked .007 and they crushed birds at all stations while a second set with chokes .010-.014 chip birds like crazy. On paper the open set should be the more open set but when you pattern them the more open set throw much tighter patterns.
So even knowing the choke constriction does not mean for certain what they will produce. My shooting partner for years refused to ever pattern any guns. He worried that what he saw on paper would undermine his confidence and thus he would not shoot the gun as well. To each his own.
