Take it to the range and test the pattern of whatever shell you intend on keeping it loaded with. Distance should generally be the furthest shot possible in your house, and ideally you should be able to put all the pellets on a man sized target from that distance. How big or small you'd like your spread to be is up to you, but I'm of the opinion that a tight patterning 8 pellet 00 buck shell is one of if not the greatest loadings for defensive shotguns