Most reloaders are guesstimaters, one turn of the die is about .071 thousands, 1/8 turn is about .015 thousands, .015 thousands is .001 over a field gage, a gap of .015 is neck sizing and if resistance is felt partial body sizing without shoulder set back, unless I am sizing cases for my Eddystone M1917 30/06, the rifle has .016 head space, in this situation I am full length sizing the neck and body with .001 shoulder set back.
Redding makes shell holders with different deck height (5 for $60.00), anything the Redding shell holders can do can be duplicated with a standard shell holder and a machinist feeler gage. Is there as much difference between dies of the same manufacture as there is between chambers in a particular caliber, then consider the difference between manufactures? When the shell holder and die are adjusted for sizing .000 (zero) space between the shell holder and die with the ram up is full length sizing? A full length size case measures .005 shorter from the head of the case to the shoulder than the chamber from the bolt face to the shoulder of the chamber (in a perfect world). What if the the shell holder/die is not perfect, what if the chamber is not perfect? I have always been of the opinion threads on my press and dies are there for that reason, and, I have confidence in my presses.
Again a standard RCBS shell holder, press and die is capable of sizing cases .017 shorter than a go-gage size chamber in thousands all the way up to .011 over, with the use of a feeler gage.
Lee shell holders are capable of sizing cases .005 (..020 shorter than a go-gage chamber) shorter than the RCBS shell holder. Other shell holders were made with a precision fit, those shell holders had their own advantage.
Seating and shell holders, one shell holder does not offer an advantage over another, the die is not adjusted down to contact the shell holder when seating, and because the seating die does not contact the case when the ram is raised the shell holder either floats the case for centering or the shell holder center the case when seating, the advantage goes to the tight shell holder.
F. Guffey