I'm a little afraid to jump in at the cost of further confusing this gentleman needing help, but here it goes. I wont try to solve your problem as it seems like you have some good advice above but I do want to clarify thing that is certainly confusing.
Datum point: ill jump past the technical definition and straight to the fundamental principal. The tool has a hole that is large enough to fit the case neck and part of the shoulder through it, but stops at some point on the shoulder where it is just too large to fit through the hole. That point is essentially the datum point. Its just a point that the tool stops due to the shoulder being too large to slide in anymore, and it will always stop at the same point on every piece of brass. The numerical reading as far as the numbers go, really don't matter. All that matters is that you are able to get a reference number so you can then calculate how much you want to reduce it by and get that reading once you size it.
The reason why what the measurement reads doesn't matter is because everyone's tool may not read the same. When it is measuring in thousandths or less you may get differences between tools due to manufacturing tolerances. However, whatever the tool reads it will read each time consistently. So, for instance you and I wouldn't be able to match our cartridge base to datum point measurements up but we could verify that we are reducing the original number by say .002 and it would be accurate to say that even though we started out at different numerical numbers. Likewise, the readings you are used to seeing may be different if you replace the tool with the same exact kind in the future. So if you wanted to look up a reading you got some time ago in your data book when you were using a different measuring tool, and wanted to get the same measurement using a new tool, you might have to apply a 1 or 2% error to calculate what your new micrometer should be reading in order to get the same result.
It would be a hell of a lot easier if you could zero the tool out like on a run-out gauge on a lathe or something. I hope I haven't caused too many technical falsities in an attempt to shed some light as to why this in particular is so confusing to new reloaders. Jesse
Datum point: ill jump past the technical definition and straight to the fundamental principal. The tool has a hole that is large enough to fit the case neck and part of the shoulder through it, but stops at some point on the shoulder where it is just too large to fit through the hole. That point is essentially the datum point. Its just a point that the tool stops due to the shoulder being too large to slide in anymore, and it will always stop at the same point on every piece of brass. The numerical reading as far as the numbers go, really don't matter. All that matters is that you are able to get a reference number so you can then calculate how much you want to reduce it by and get that reading once you size it.
The reason why what the measurement reads doesn't matter is because everyone's tool may not read the same. When it is measuring in thousandths or less you may get differences between tools due to manufacturing tolerances. However, whatever the tool reads it will read each time consistently. So, for instance you and I wouldn't be able to match our cartridge base to datum point measurements up but we could verify that we are reducing the original number by say .002 and it would be accurate to say that even though we started out at different numerical numbers. Likewise, the readings you are used to seeing may be different if you replace the tool with the same exact kind in the future. So if you wanted to look up a reading you got some time ago in your data book when you were using a different measuring tool, and wanted to get the same measurement using a new tool, you might have to apply a 1 or 2% error to calculate what your new micrometer should be reading in order to get the same result.
It would be a hell of a lot easier if you could zero the tool out like on a run-out gauge on a lathe or something. I hope I haven't caused too many technical falsities in an attempt to shed some light as to why this in particular is so confusing to new reloaders. Jesse