Chet,
Where I live we can't use scopes in muzzie season so all I ever shot that mk 85 with was iron sights. It shot as good as any other guns I have but I never tried shooting 200 yards. I can't see that far. With that gun I found that the Knight black sabots worked good and I didn't use as much powder as you are using there. About 95 grains of 2f 777 shot pretty darn well. From the targets you have shown on here I'm sure you will have it shooting better than I ever did.
Art