I suggest setting up a target out 25 yards, for your first shot. Make sure you are shooting from some kind of rest, this ensures an accurate reading of your shots.
If you hit high don't worry about it. If you are certain it's shooting okay left and right, move target out to 50 yards, and shoot again. If you need to make adjustments left and right, do it at the 25 yard mark. Move rear site left if it is hitting left, or move it right if it is shooting right.
I don't know if you have windage and elevation on your rear site or not. Just remember if you elevate your rear site you will be raising the elevation of your slug.
A slug will rise as it comes out of the barrel, so don't worry if it is shooting high. Just see where it hits every 25 yards. I would only site it in, out to a hundred yards.
Then shoot it at 125 yards and see where it is hitting. Then just adjust your aim, if you need to.
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