I originally bought the Forster concentricity guage. It worked OK, but............ I then borrowed a RCBS Case Master from a friend. It does so much more for not a lot more money that I bought my own about a month later, and never looked back, as the saying goes. It's also easier to use.
Also, another book you might be interested in "Precision Shooting Reloading Guide".
If you're reading this, thank a Teacher.
If you're reading this, in English, thank a Veteran.
C'ya, John. (LRH #16)