For a 12ga, I use a 6" metal machinist scale (ruler) like Miller does. You slide it in along the chamber wall and it will lift off the chamber wall when it rides up the forcing cone. A little finesse and you will see the light under the scale where it lifts from the chamber wall. Then read the scale. That's it.

On the smaller bores, I use the depth measuring blade that is on the back end of vernier, dial, and digital 6" calipers to do the same thing.