Your close, think of all this maybe like this.
Whichever bushing you use for the comp, the hole or bore ( is land diameter) that the bullet fits in, is the top of the lands ahead of the chamber and in front of the bullet seat and chamber proper. The bushing travels on the sliding part of the mike and simulates differences in how the angle of that leade is tapered and or formed. And it gives a repeatable reference point. Even if the bore of the comp bushing is smaller than the caliber. The bullet will slide in and out of the case at your will.
The chamber, between two different rifles of say the same caliber but different makes had differing tooling and wear variables possibly to make the lands interfere at differing points along its length with the ogive of a bullet because its a radius. The two chambers, may be exactly the same size and length, but its the area in front of the neck or bullets seat that needs to rapidly taper to bore diameter and cut the lands at the same time for bullet clearance. Its a cone shape with a short cylindrical section. And very important for quality of chambering.
The difference in diameter at the top of the lands is the OD of the neck of the cartridge, tapering rapidly to diameter of the depth of the grooves to the diameter of the bore and removing just enough of the lands on an inclined angle to allow the bullet to fit up in there without having to travel unsupported for as little as possible. And quickly seal the bore, nice and straight and make all the area thats not a case temporarily an extension of the case till the bullet moves forward and the neck is blown tight against the chamber making a seal.
Forget about overall length that includes the tip of the bullet to a certain extent. You could grind them off or clip with pliers and still get the same length measuring with the tool. Start thinking of measuring from the portion of the bullet that curves and transitions to its diameter, and the length of the case and where the bullet ogive measures up once you seat them.
Those tools with a couple of bushings will fit many rifles becuase they usually come in a set and very few if any identical rifles will have the same measurement at the lands because of production methods. As do, different makes of jacketed bullets including the cast ones we make ourselves becuase they have different forms.
What some people will do is make a dummy cartridge up, cut slots in the end of the neck to minimize tension but just have enough, seat a bullet, smoke the bullet with candle black and then fit the bullet in the neck at just the point you can see the smudge removed from the blackened bullet where it contacts the lands. And maybe back off a little for errors in case length or methods of consistency when bulleting a case and or loading.
