Next is the shell plate final adjustment. The screw in the center is tightened until it is tight, nothing can move. Then it is loosened just enough so that the shell plate can rotate freely, without binding, about 1/8 turn. Finally and very importantly the set screw for the shell plate is tightened. If it isn't the shell plate will eventually bind up and not turn.
Both steps are shown here.. The shell plate is checked for free rotation, ok. Then it is checked for rocking, ie how much up and down movement it has, about 0.015" according to a dial indicator. Dillon states that this is ok and not critical. Any tighter the shell plate binds up.