Bull N. My favorite trim/forming die is the 308 W, if I had one trim/form die it would the 308 W die.
The trim/form die is not a finish die meaning after using the trim/form die the case must be sized with the full length sizer die. I will assume you sized the case first then used the trim die. As to trimming to length, I use the 308 W trim die to form/trim 7.7 Japanese when forming 7.7 using 30/06 cases, for those that can keep up that is a large gap between the shell holder and bottom of the die (.312, starting with +.010 to allow for chamber length).
The trim die must be adjusted to .000, that would be adjusting the die down to remove all the slack in the press, threads and linkage to get consistent results. If I had the chance to start over I would have a trim/form die for every chamber I load for, again the 308 W trim die fills in gaps for trim dies I do not have.
Again, to avoid moving the shoulder back more than necessary the reloader should know the length of the chamber, again (reminder) the form/trim die is not a finish die, after forming a case the case must be sized in a full length sizer die, and no, the case may not require full length sizing if the reloader knows the length of the chamber from the bolt face to the shoulder of the chamber.
Mix and match: there are differences in shell holders by design, the Herter's shell holder has an advantage over modern? shell holders, wasted on modern reloaders, the deck height is .125, measuring the deck height of a shell holder can be accomplished with a dial caliper. The difference between Lee and RCBS, the lee has a loose fit, again, that does not mean anything to most, for me it allows for sizing cases an additional .003 thousands for short chambers.
F. Guffey