If you are planning for a bullet that has a long ogive, to be sure that there is enough of it in the neck (or a blunt one, to be sure that its base does not fall in or below the neck shoulder junction), looking at the print in the manner that you have described can be important. As you undoubtedly are aware there is a gap between the end of the case neck, and the start of the angular transition from the neck portion of the chamber to the portion of the throat that is parallel sided and slightly larger than bullet diameter (the freebore, often referred to dimensionally as the throat length). Forward of that section, the leade angle intersects the ogive of a bullet that is seated to touch the rifling or be engraved by it. It is the distance from the end of the case to where the bullet contacts the rifling that is of concern when designing a reamer so that it is optimized for a particular bullet shape. You are correct in your assumption that trim length, chamber length to the end of the neck, the length of the transition, and freebore length all are part of this calculation. There are other components as well. The leade angle, and the the point within which the barrel headspace falls within the allowable tolerance are also involved, but to a lesser degree, the former because leade angles have become pretty much standardized at a degree and a half per side, and the latter because the probable variance is small.
The best way that I have of looking at this problem is by assembling a dummy round with the bullet being considered, to a length that is longer than what would have the bullet just touching the rifling. By first measuring the combined length of the case in a "gizzy" (short piece of barrel partially chambered with my reamer) and then taking the same measurement with the bullet in the case, and subtracting one from the other, I can see how much longer than touch I have loaded the dummy round. Since I know that my reamer was ground for a maximum case length of 1.515", with a freebore of .060, I can get a pretty good idea of what will be needed to accomodate a given bullet so that it is in the position within the neck that I want. The only situation that this will not work for is one where the bullet cannot be seated longer than touch. So far, although there are a couple of bullets where the amount of shank in the case neck would not be usable for actual shooting, I have not run across one that could not be loaded in a dummy slightly longer than touching so that it was secure enough for measuring purposes. Having looked at every bullet that I have used and been able to get as sample of in a manner similar to what I have described, and given that my current taste runs toward the double radius ogives that leave less bullet in the case neck than older designs, and that I have gone to larger neck clearance, I would probably order my next reamer with a shorter freebore, perhaps .025, with a neck diameter that was slightly larger, to restore case neck thickness to what it was before I thinned them, perhaps a .264, which I believe would preserve the option of trying the newest Norma brass, which has thinner necks than Lapua.
