I just answered this same question in the current "Chamber Length" thread, and explained why I much prefer Miller's oft recommended use of a simple inexpensive 6" machinists scale over 3" x 5" cards, fancy brass tapered chamber gauges, or fired shells. You can read that reply in my post here:

https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=538330#Post538330

As you can see from this illustration of a 12 gauge chamber, the length of the chamber is from the breech end to the transition where the forcing cone begins:


A true sign of mental illness is any gun owner who would vote for an Anti-Gunner like Joe Biden.