Fired cases tell you nothing about which bushing you want. The number you first need to know is the outside diameter of a loaded neck in the brass you will be using. Decide how much of an interference fit you want and subtract that.
Example: I've turned the necks of some Lapua 308 Winchester brass to .014" thickness. Twice that plus a .308" bullet adds up to .336". Suppose I want a .002" interference fit when seating a bullet. Then a .334" bushing will do the trick. (Assuming brass with necks that aren't work hardened and springing back)
