None of my dies have the expander mandrel in them. Why pull something back thru the neck that would change your head space. Possibly pulling it to the point it will not chamber. If your measuring your fired brass and setting your dies to bump the shoulder back .003 to .004 then pulling a expander back thru it you have just undone all your work for sizing. Another part to the puzzle is dwell time in your die. Running brass in and out with a quick press stroke will also have your brass all over the place. Allow it to normalize in the die, this will also show in consistent neck tension (Assuming you have neck turner your brass) and head space. Quick stroking the press handle leads to bad things.
Why neck down past where you want to be then to just expand back. Brass Work hardens. The brass will form to the bullet as long as your Neck ID is smaller then the bullet OD, and walla your Neck is round to match the Bullet! Don't make things harder then they appear.
My answer to you is to dump the expander. Neck wall thickness X2 plus your bullet diameter minus .003 to .005 depending on how much tension you want.
Pick your bullet, primer, and powder charge, and tune it with your seating depth. Every load will shoot with the proper seating depth, even factory ammo.