Extended range sessions with a slug gun don't do you any favors. You noticed you are flinching, so you really can't tell much. Just sending $2 downrange with every shot at that point. It may help to use some sort of pad on your shoulder right from the start to minimize recoil. I have seen guys shy away from the gun at the range because they are afraid of recoil, but that only makes it worse. That gun needs to be firmly in the pocket of your shoulder and your cheek needs to be down on the stock.
I sight slug guns in for 2" high at 50 yds. Then I verify elevation at 100. I don't shoot more than a couple at 100. I have a 12 Ga Marlin Slugmaster bolt gun. It does not shoot higher velocity slugs well at all. It likes them in the 1300-1600 fps range. Brenneke KO Sabots are great in this gun. The Federal Fusion 7/8 oz sabot at 1600 fps looks good, and I may try them next year. They are the cheapest sabot around here at $9.99 a box.