Scruffy's sight you shotgun in using a 5 shot box instructions.
On my ultra I attach the harris br bipod on the front swivel and use a 'v' block (cut from 2"x4") under the butt stock to the rifle is fully supported. At 25 yards I position the crossairs on the bullseye by moving the block in the rear to control elevation and windage. When it's "aimed" and just sitting on the bipod and block I grip it snuggly and squeeze the trigger. BOOM. Then I adjust the shotgun again so it's sitting on the bipod and 'v' block and the cross airs are centered on the bullseye. I then very carefully adjust the scope so the crossairs are centered over the first shots impact. Now, for the first shot at 25 yards, POA (point of aim) and POI (point of impact) match exactly.
Then I back the target out to 50 yards. I fire three shots using the bipod and block for total gun support. After the third shot at 50 yards is fired I once again carefully place the crossairs on the bulls eye and with the shotgun supported on the bipod and 'v' block adjust the scope so the cross airs move from the bulls eye 2" below to the middle of the 3 shot group. I go put the cross airs 2" below the 3 shot group so POI is 2" above POA. And if your scope's centerline is 2" above the bore's centerline a 25 yard zero for elevation will give you almost 2" high at 50 yards (slug travel 2" from centerline of bore to centerline of scope in the first 25 yards and travels almost another 2" from centerline of scope to just under 2" high in the next 25 yards).
Then I move the target out to 100 yards and fire the last of shot of the 5 shot box to verify it's 100 yard placement. If you are shooting slower slugs POI will be lower at 100 than the faster high velocity slugs. Some of the high velocity slugs may arc up to 2.5 inches high around 75 yards and be around 2" again 100 yards. While the slower slugs may be back around zero at 100 yards. And if that one shot is right on you can start shooting for groups or move the target out farther. If it's not, the shotgun either moved while you were adjusting it, you had a flyer, etc.
Also, during this process I check the ring screws after every shot to be sure the scope and rings are seated in tight.
For me, even after the 5 shots to sight in are complete, I like to shoot another 5 to 10 shots just for my own confidence level in the shotgun loads accuracy, scope setting, scope position (eye relief), etc.
later,
scruffy