I would definetly buy some 2 3/4 inch turkey loads(cheap ones) and start my sight in with them...

I just did this last week end. I had a bunch of win XX 2.75 turkey loads laying around. Set a target up at 10 yards and sighted my gun in on a 1" bulls eye until my pattern was centered... Then I moved out to 30, then 40... Once I get my pattern were I want it I put the expensive shells in and shoot a couple....

No matter how you do it your going to have to bite the bullet and shot at least 3 to 5 of those expensive shells