I tend to subscribe to the K.I.S.S. principal whenever possible (keep it stupid simple), more often than not, because as I advance in age, I feel like I'm getting stupider.
With that in mind, we are using the sensation of touch to "feel" when the bullet is in contact with the lands, regardless of the method we use, we seek to measure that distance from the shoulder of the cartridge case so we can reliably seat a bullet at a given relationship to those lands.
So the two relative datums are the lands and the shoulder of the cartridge, unless it's a belted cartridge. Later we'll measure the CBTO to allow a practical measurement during seating operations.
So imagine holding a barrel not installed on an action, or just a piece of a barrel, the chamber, throat and about 1 inch of lands, that's all.
You place your ready to fire cartridge in the chamber with a firm push, and then remove. It will either stick because the bullet is in the lands as you try to remove or it will remove easily. If it sticks, you can seat the bullet a bit further in the case and test it again until you find that position where it just fails to want to stick upon removal. Presuming the throat and barrel are free of any fouling, this would be your touching lands dimension.
Now, instead of using a ready to fire cartridge, use a properly sized and ready to load cartridge case that has been drilled and tapped to fit a rod. Could even be drilled to fit the Hornaday tool. I prefer to use a section of 3 piece 6-32 cleaning rod.
And now with bullet seated push this hot dog on a stick looking contraption into the chamber of a barrel installed on an action and repeat the test. What you have now done is to eliminate all the extraneous moving parts of other processes, and reduced it down to the element of the human sensation of touch.
The whole process takes about 5 minutes, less if you have a selection of pre drilled cases in the chambering of your choice. Best of all, you can keep your just created case with bullet seated to touch as a Tool for setting up dies, or checking throat erosion, or use it to compare CBTO to the next lot of bullets. Lots of uses for that little tool.
I hope this helps.