va connoisseur, if you haven't already I think you would do good to read this thread:
https://thefiringline.com/forums/showthread.php?t=593684
What I have found so far is that fired cases often have different measurements. If you are to rely only on the fired cases for a measurement I believe it would be best to fire high velocity factory ammo from the same box so start there for consistency. That said, I just measure three factory fired cases that were high velocity that I used for fouling shots. One was .001", the next was .002" and the next .003" from the zero mark on the mic. I then measured 10 cases that had been custom full sized and then fired. Every one of them was .0025 from the zero line. After sizing them again they were all at the .002" line. This tells me that firing them after custom sizing only moves the shoulder forward .0005" and they have become very consistent. The ten fired rounds were from a 10 shot ladder with each being a progressive powder charge.
Do not trust that the zero of the mic is where the manual says it is. Many people have found the zero to be off by a few thousandths. Take it to a gunsmith and ask to borrow a go gauge to verify where it is reading.
If you really want to know for sure what your headspace (bolt face to shoulder/datum) is you must measure it. There are several ways to do this. I made a useful tool out of one that was useless. I took the ogive measuring bullet from the RCBS precision mic kit and used it to make a chamber headspace measuring tool:
I took a full sized case.
I then used a dremel tool cut the body of the case off of the shoulder just behind it leaving a sized neck and shoulder.
I used a polishing bit to take off any roughness in the metal left from the cut.
I unscrewed the bullet out of the plastic body of the tool.
I then took some masking tape and cut the end square and wrapped the end of the plastic body of the measuring tool with the tape about four times and cut off square so that the end was just short of overlapping where I started the wrap.
I then carefully pressed the case shoulder over the tape. Depending on the thickness of your tape you may need to wrap a different number of times to get it tight but not too tight. Got it on most of the way but not quite...
I then removed the expander ball and primer punch out of my full sizing die.
I tightened the allen screw in the bottom of the tool as tight as I could get it.
I then ran the tool into the sizing die until it pushed the shoulder onto the tool just enough that the plastic body was pushed to the rim of the neck. This was to insure that the shoulder was concentric.
Viola!
Depending on the type of rifle you own there are components to be removed from the bolt so that there is nothing pushing forward on your case head on the bolt such as extractor pins and so that the cocking mechanism is disabled. You may then use the tool to measure from the bolt face to the shoulder/datum. Do not over tighten the tool or it will give you an erroneous reading that is long. You should be able to slide the body back and forth with a little force. Measure three times until you get a consistent length.
Now you have a real number to work with. This number may differ from what you have fired through your gun because the brass you fired may not be formed fully to your chamber and also because the shoulder angle of your chamber has a slightly different angle than a sized case. So if you are measuring a fired case and a sized case to fit your chamber you are going to be off a bit.
I am using my tool to find out where to set my die to fully size my brass so that the shoulder is sized to where it is .001" head clearance. I use a feeler gauge to put the correct gap between my shell holder and my sizing die to do this. If you plan to partial size your brass you really have no need for this tool because the brass that is partial sized will not be consistent in length or width. When I say partial sized I mean neck sizing with the full sizer die.
I suggest that you use your RCBS precision mic and run your die out 3/4 turn and partial size some brass and turn the die 1/8 turn and size and keep doing that unitl it is fully sized. Don't forget to keep lubing! Measure after each time and watch what happens to your shoulder length. Then take 5 rounds that you have fired and partial size them all with the same die setting (like you would if you were going to partial size for your gun) and measure each of them. I think you will see why I no longer partial size my brass.
: see where your fired cases measure, set up your resizing die so you are slightly below that mark on the gauge.