Check out these two sites...the information on them should give you an excellent basis for understanding reloading.
I just finished a 6mmBR and another poster needed information on the same bushing dies...do a search...most of the information you need is scattered all over www.saubier.com
, small caliber discussion board and several others, including data and a target from my BR.
It is not quite as simple as measuring a fired case but almost...You need to remember brass springs back a certain amount and it isn't always the same. The best way to determine which bushing and a way to cover all bases is to measure a fired case...measure the neck...seat a bullet into the case then measure it again (you can seat it by finger pressure usually. That will give you the amount of "spring back" or close to it.
You also want to measure the ID to determine the difference between bullet size and neck ID of the fired case...You want also measure the neck walls. Do the measurements with a bullet in the case...DON'T just depend on calculations because they will jump up and bite you. Take it from me...I've used bushing dies of one sort or another since before Methuselah was a baby and if you are dealing with a possible tight neck
you need to be accurate.
Basically...a fired case will give you the chamber neck ID minus the brass spring back. You want to have about 0.002-0.003" clearance for bullet release for a varmint rifle and maybe a 0.001" less for a tight neck chamber for a target rifle...all argumentative and subject to the whims of the rifle. Seating a bullet will give you the OD of THAT brand of brass and the difference between the fired case and the seated bullet case will give you the clearance for THAT brand of case and bullet for THAT rifle chamber only. Once you have all these measurments you can decide which bushings to purchase.
If the difference between the fired case and the seated bullet case is "normal"...i.e...about 0.0015" spring back, the seated bullet case being the larger...I would buy bushings 0.001", 0.002" and 0.003" smaller and 0.001" larger than the fired case measurement. That will cover most of the contingencies...bushing size varience, difference in measurements caused by out of spec miks and so on. It will also give you some wiggle room if you need/want to neck turn.
One slightly cheaper method and one that gives you more options is to use the non-match Redding type S bushing sizer and then order a Forster Ultra seater for the 6mm BR...you can then use the Ultra seater for other calibers and cartridges by just ordering a slider and seating punch for the right caliber. Get a Forster cataloge and check out the way the Ultra seaters are made...that should give you the idea of "interchangeability". Twenty bucks for a slider and 10 bucks for a bullet seating punch is cheap to get a benchrest quality micrometer seater.
You have an excellent toy to play with...take your time to enjoy it.