Sighting in a slug gun is a pain due to rainbowlike slug trajectories. The method I’ve come up with that’s least wasteful of time and ammo is as follows: First, sight it in to hit 2 inches high at 25 yards. Next, see where it shoots at 50 yards; you should be either dead on or a few inches low. Move the target again, out to 75 yards, and see where you hit there. If you’re too low, you’ll have to adjust the sight so you’re just about dead on. Then recheck where it hits at 50 and 25. You should be able to reach a point where you’re no more than 3 inches high or low at all three distances.
After I have spent a few hours getting the tar knocked out of me by firing huge lumps of lead, I find a quiet place on the range and have a good cry. You may find this helpful, too.