Basic shape of the pattern looks fine. Next time you go out take a piece of paper that's at least 3X3 feet, And put a large black dot in the middle of the paper and use it as a aiming point. As far as the range goes I will use the same 15 yards that you used this time. As a rifleman myself, let me remind you that your eye is the rear site. When you first pattern shotgun you are trying to determine where the pattern is in relationship to your front site. With an older shotgun like yours, I would expect that half pattern to be above the spot and that half pattern below the spot (50/50), and I would also hope half of the pattern to the left of the spot and have half of the pattern to the right of the spot. If this is the case, you can try moving back to 25 yards, and if it's still good try moving back to 35 to 40 yards.