Well, I suppose it is good advice to own calipers and learn how to use them. Here is my problem with them: I can take 5 measurements on a mating surface (nib, finial, CB, etc.) and get 5 different numbers. They will be close, but not identical. Likewise, measure the material at the end of a pen blank 5 times, 5 different numbers. Even slight changes in finger pressure against calipers will marginally change the reading.
I've found that relying on a good set of John's cnc bushings produces better results for me than I can get with calipers. If the blank feels seamless and smooth against the cnc bushing, it will fit perfectly with the pen parts.
Still, for the simpler task of figuring out which bushings go where, calipers are a quick and easy solution.