not meaning to sound snarky or anything but sometimes its heard to measure a loaded round if you don't have the dies to make one or don't happen to have one on hand.
ive been in a similar situation with a different calibre.
what i can suggest is to measure the material thickness of the neck of your brass.
don't measure just one casing, check as many as you can in order to get an average.
if its lapua brass your working with than they are usually pretty consistent.
now take that number and multiply it by two.
that number is the total case thickness around the neck on your specific brass batch.
now measure the diameter on several of the bullets you are going to load and average out.
combine the two numbers together (total neck thickness and bullet diameter).
this is the overall thickness of a loaded round, subtract from this number 0.001 or 0.002 (depending how much neck tension you want to create) the resulting number is the bushing diameter you should decide on.
hope this info helps
math example:
2x brass neck thickness+ bullet diam'= loaded round thickness=> subtract 0.002 or 0.001 =desired bushing size
