Good video, Gil. Proper and consistent gun mounting is the foundation of building a solid and successful shotgunning program. But, shooting the gun at a plate or paper is the only definitive way to tell if the shot load will go where you are looking. Holding off to one way or the other to compensate for a gun that is not regulated is horrible advice, but has actually been suggested on this site recently.
I have my own steel patterning plate for such exercises, and generally mount and shoot offhand, in order to predict field or range results. However, If I find that a barrel's POI is not correspondent to it's POA, I will go to the sitting position, just like sitting rifle shooting, and test it that way to be certain I was not in error with my offhand shots. I don't like the idea of benchrests or sleds for shotgun pellet load patterning.
I can assure you that if you go to a pro gunfitter and he has you shoot the plate with the try gun you will not be shooting off a rest.
JMO, YMMV
SRH
