This, plus headspace is determined by the rim of the case and its relationship with the bolt face, as with all rimmed cartridges. This data, while interesting and appreciated merely proves the sloppiness (or lack thereof) of the rifle's chamber. Sloppy headspace can be corrected by switching the dynamic from rim-based to shoulder fit (as with a rimless case), but that is achieved after the first firing and then not touching the shoulder with the sizer die like sqweeler said. The alternative, with new brass, is to expand the neck to a larger diameter and then re-sizing to proper diameter leaving a tiny adjunct shoulder that effectively stops the case from entering the chamber too far.
That's all well and good, but remember too that once bad headspace has been corrected for in this manner you are ending up with the case protruding back out of the chamber farther before firing, leaving just that much more of it unsupported by the chamber walls.
