Using your barrel as a gauge to determine case overal length for reloading

I always have a hard time finding this image, which very clearly shows how to use a barrel to determine the optimal COAL (aka OAL) for ammo to be loaded into your specific gun.

I'm not sure who made it, to give them credit, but I found it at http://img291.imageshack.us/img291/9471/45seatingpossibilitiesxyn4.jpg

I'm mirroring it here in case it ever goes away from ImageShack.