If the holes overlap measure outside to outside of the holes farthest apart and subtract one bullet diameter.I use all shots fired in every group and do not delete any called flyers.When you average out several groups a single flyer has little effect.I quote a guns accuracy by the average of several groups not by the best group fired.A gun that averages 1/2" for several groups deserves to be called a 1/2" gun.A gun that fires a single 1/2" group but averages 1" when all groups are taken into account is a 1" gun to me.Using this criteria there are not nearly as many 1/2" hunting rifles as one would be led to believe.