While you can wing it when partially re-sizing brass that is to only be used in a single gun, the "correct way" takes all the guess work out of the exercise and assures you get consistent result which can lead to better accuracy.
You need the RCBS Precision Mic to measure the fired cases that come out of the gun and to set up the sizing die.
You randomly select about ten case that are just fired and measure the distance from the shoulder to the base of the cases using the correct parts of the RCBS gage. All cases should be within a couple of thousandths of each other. What you want is the partially sized brass to be one or two thousandths (the gage is marked in thousandths) SHORTER than the average of all the cases you measure. That assure you will not have any trouble closing the bolt on a loaded round.
Set up the sizing die to absolutely not touch the shoulder of the case. Then adjust the die down using the RCBS gage to measure each try. You want to get to the point that the gage reads the one or two thousandths (the gage is marked in thousandths) SHORTER that you sample of ten cases. It is size, measure, adjust until you get it right. Then lock down the sizing die. Remember you cannot trim until after you have sized the cases.
This is a much better approach than to neck size with a neck size only die as the case body is brought back to the correct size for easy loading in the rifle and the shoulder is close to the end of the chamber (by one or two thousandths) for better centering of the case in the chamber and ultimately better accuracy. When you use a neck size only die eventually the cases will become hard to close the bolt on because the shoulder can eventually grow to the point that the bolt is hard to close.
This is the procedure recommended by excellent reloader journalist like John Barsness. He even has a video that describes this process and others that help to get better accuracy.