Some folks figure bumping the shell holder squares the die with the shell holder and so with the case in the shell holder. Some folks square the case head to the case body too.
The numbers on the shell holders reference the .125 normal shell holder. The shell holder does not itself bump the shoulder of course but allows the die to come down the additional referenced distance. Given spring back and cam-over issues the results may not be a perfect match for the numbers.
I believe the competition shell holders to be a quick and generally successful way to arrive at a final setting with fewer trials and less scrapped brass than other systems of gage and turn a little bit but I don't believe the final result is any better. That is the Redding Instant Indicator - left set up in another press for ready reference - and the Redding competition shell holder set give a quick measurement and a quick adjustment at some extra cost for the convenience - and again all measurements are relative for the dial indicator and shell holder.
