We all sight in our shotguns to be 2" high at 50yds. this should put you about dead center at 100yds. What slugs are you using? We use Lightfields 2-3/4" Hybed EXP, but your shotgun may like a different brand. Look at:
www.lightfieldslugs.com They give some great info on sighting in, and why you should sight in your shotgun at 50yds.