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You might want to adjust the sights to be right on at 50 yards. The difference at 15 yards will be far less than 5 inches off. This approach eliminates the need to compensate under potential high pressure situations.

Given that the sights are above the bore axis the projectile comes up to meet the line of sight (see the bore axis in the picture) at 2 distances (in your case first at 15 yards) then as gravity takes over the projectile starts to drop to cross the line of sight at some greater distance. Extend your target out to 75-100 yards and you will find that the slug does intersect, then fall below, your sight picture some where beyond 50 yards.