My merthod is to use a 6" machinists acale. One of the flexible 6" scales available at home improvement centers for $2.00-$3.00 will work just as well for this. As a retired machinist I just happen to have a Starrett. Look through the bbl from the breech, held up in front of your eye toward an indirect light sourse. An uncurtained window is a great sourse. The forcing cone will appear as a shadow. Slide the scale in alongside your eye contacting the chamber wall. When the corner of the scale just touches the shadowline STOP & mark at the breech, then read chamber depth directly from the scale. With just a small amount of practise this can be a very accurate method. Many older chambers have some variation of actual diameters. I have personally witnessed a chamber which was only .001" smaller than the end of a chamber gauge show that chamber to be more than 3/16" short, IE a 2 3/4" chamber read 2 9/16" on the regulation chamber gauge. In this case the scale was more accurate than the gauge.
Personally I have found this method of watching the shadow line to be both easier & more reliable than trying to Feel an indistinct break between two tapers.
