Not sure why you are getting that odd trajectory.
I would dial it in where you want it at 100yds (dead on?, 1" high?) then see where it shoots at 50, 150, etc. and adjust from there.
If it was me, I'd be most concerned with dialing it in to your expected range. I'd bet that most shotgun slug shooters probably shoot under 125yds the majority of the time. If you expect most of your shots 150-200, sight in accordingly.