Robert, I would be referring to distance from the aiming point, for each barrel. If each barrel of a S X S puts it's pattern centered on the aiming point, horizontally, I consider it to be perfectly regulated for the load in use. The vertical can be dealt with fairly easily, to a reasonable degree.
While the lasers are interesting, and provide good and solid information, the results ignore the effects of recoil, which is why there is convergence built into S X S barrels. And the amount that things like loads, and how tightly you hold the gun, affect where the patterns print, too. The lighter weight the gun, the more the effect, in my experience.
SRH
