While the process you described matches up with the Hornady instructions for the tool, it is greatly flawed. You'll note, even in the Hornady instructions, they recognize there is an inherent difference between the headspace length of the Modified Case products and actual headspace lengths appropriate for real-world rifles - this creates an offset between the length you are reading on your tool and the REAL length you need to be using for your loads.
The Hornady system of tools does not tell you your "JAM" distance, at all, as it does not actually dead-length against your bolt face. When you insert the modified case into the chamber, the case dead stops against the chamber shoulder, and when the bullet spindle is used to push the bullet forward, it dead stops the bullet against the leade.
This is critical information, but it is absolutely NOT relevant to the distance between your bolt face and the datum line where the bullet would meet the leade. The system of tools ONLY tells you the distance from the datum line of the case shoulder where it headspaces against the chamber and the datum line where the bullet ogive meets the leade.
A reloader then has to determine the appropriate headspace length between the bolt face and the datum line of the shoulder.
A guy simply has to correct their measurement by comparing their modified case against a once fired case (ideally a twice fired case, after the 2nd firing, before resizing).
All of the Hornady tools have to be used in congress to pull this off, the OAL Gauge spindle, the Modified case, the Bullet Comparator bushing, and the Headspace Gauge bushing. Otherwise, you're not actually getting any real measurements of your actual chamber.