You need a reference point to start and below I use a GO gauge and this is "zero".
New unfired .243 cases were minus -.002 to -.009 (shorter than minimum headspace or GO)
A fired case was plus + .002 (longer than minimum headspace or GO)
Your fire forming is very important because a new cases above can have .011 head gap clearance or air space between the bolt face and the rear of the case. The head gap clearance can cause the case to stretch in the web area as seen below. Seating your bullets long and jambing the bullet hard into the rifling will hold the base of the case against the bolt face, and prevent stretching.
Once fire formed the case is the same length as the chamber and can be neck sized only until they become hard to chamber. At that time you would bump the shoulder back .001 to .002 with a full length sizing die