How about either actually sighting in in for dead on at 100 and forget about the 25. Or, even better, find a quality sabot and look at the ballistics. For most of them, if you sight it in 2 inches high it is dead on at 150. Although some people claim great accuracy at 200 w/ a 20 slug gun, the two we have are great to 150, past that things start to fall apart.
Just don't shoot cheapie lead foster or other bare lead slugs in the barrel. Stick to sabots. At first, the gun will appear to digest the lead slugs fine. But they are very soft and will heavily lead you rifling and accuracy will go south fast. Spend the time and money on quality ammo and the gun will be great.
Hank