Note that the case neck brass hardness will also determine how much spring back it has from being sized down. Over the years of using gelded full length sizing dies (no balls), the die's (bushing's?) neck diameter should be whatever lets the bullets stay in place when in the rifle's box magazine and the round in the chamber's fired. If the ammo's going to be single loaded, then less neck tension is fine. Depending on the case neck wall thickness and the brass, it is 1 or 2 thousandths smaller. You'll have to pick the bushing size that best suits your objectives.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master