Don't know your cartridge but generally with fireforming (esp. when shoulder position or angle is changed) you want a "medium" load (not mild and not max) of a poweder that is on the fast side for the cartridge in question and a bullet that can be seated into the lands - the idea is that the bullet holds the case head back against the bolt and any gaps at the shoulder area will be filled when the cartridge fires.

This is the process I use when fireforming 7mm TCU, 375-06AI, 22-250AI, etc.


Now for straight neck up/neck down in caliber (i.e. 30-06 to 338-06, 6mmBR to 7mmBR, etc.) just start doing load development - no special procedure required.