Mike - Bob Green's comparator tool effectively measures from the seating die contact point to the caliper insert contact point on the bullet ogive. In other words, it measures from where the seating die stem pushes the bullet in, to the point on the ogive where we actually measure seating depth. Sorting bullets with Bob's tool allows you to achieve extremely uniform seating depth without having to constantly adjust the seating die micrometer. That's all it does.
Measuring and sorting bullets by other dimensional aspects such as BTO, OAL, etc. certainly has plenty of fans. However, the reason(s) for for using those approaches is typically not to obtain uniform seating depth, rather they are used to generate a uniform amount of shank in the case neck, consistent OAL for pointing purposes, etc.
Bob's tool allows one to sort bullets based on the two closest contact points that are critical for uniform seating depth. The method you described above is going to measure bullet base to seater stem contact, or at least from the bottom of the bearing surface to the seater stem contact point. That means you are including extra regions of the bullet (i.e. bearing surface and/or boattail) in your measurement that are
outside the seater die and caliper insert contact points that critical for uniform seating depth...regions that increase the chances for bullet length variance
outside the critical region. Any time you include extra regions of the bullet in a measurement or sorting approach, you lessen the accuracy/precision of that measurement, or effectiveness of that approach. The method you described above may work just fine...
unless there additional bullet length variance in the bearing surface and/or boattail region of the bullets being sorted.
