If I read your first posts correctly you were using a fireform load that separated upon firing in which case you likely had little control over the headspace. If you want rounds that perfectly match your chamber you make your fireforming loads with new brass (so it's only stretched once and conforms exactly to your chamber.) When loading the projectile you seat the bullet long so it takes a brisk snap to close the action, and fire it. Upon firing the bullet lodged in the rifled prevents the case from sliding forward when the firing pin hits it, which then causes the case to have to stretch back to the breech face. If you're necking down to a smaller diameter you can instead create a false shoulder in the neck to do the same, but that wouldn't apply in your scenario. I've done this method with many of my wildcat rounds and not had any issues with case separations as a result as you stretch the case just once, and set your dies to just bump the shoulders back enough on successive firings so the case still chambers. Bumping it too far back causes excessive stretching on firing and can lead to the same situation you had.

You can also use the cream of wheat case forming method, but you can potentially get excessive case stretch with this too depending on how radical your final case dimensions are...