Let me know if this is already patented, and I'll take it off to avoid the lawsuit
For rimless case, 9mm/40SW/45ACP for example, I have a method here to measure the maximum COL for a bullet in a barrel. No need to do 10+ times of switching between seating die and the disassembled barrel to find it.
Drop your bullet into the chamber, so the bullet contacts the barrel lands. Use the caliper (the stick end) to measure the depth of the bullet base. Then pull the bullet out and measure the chamber depth/length itself and the bullet length. The approximate C.O.L is calculated as
max. C.O.L = [bullet length] + [bullet base depth] + [case length] – [chamber length] - [tolerance]
where the [case length] e.g. for 9mm is 0.754”, and the tolerance is the gap you leave for the variance of bullet length due to casting or PC, I set it to be 0.010” (you may find it differently). This will ensure a reliable feeding. After you seat a dummy to this COL do a plunk test, but make sure to crimp the case mouth properly. Because an un-crimped dummy round may fail the test due to the out-of-spec case mouth instead of a long COL. However, this might fool you that you had a long COL so that you would seek a shorter COL than actually needed.