adding length to the neck of the chamber, it could require a neck reamer. Then the question should be about determining the length of the neck in the chamber of your rifle, I could say simple
Back to the long neck of the formed cases, rather than mindlessly trimming the cases use the long neck to determine the length of the neck in the chamber. I modify the case with a shoulder that is set back to far, meaning the shoulder can not contact the shoulder of the chamber, 'then!' I start chambering long neck cases that will not chamber because the neck is too long, it is possible to take one measurement, but it is better to trim a little/few/thousandths in increments of .002" until the case chambers and allows the bolt to close without sizing the neck at the throat.
F. Gufffey