There are dozens of diamonds in the bronze tip. As they dull and become ineffective, they slough off, the bronze wears and new diamonds are exposed. At any point in time you might have from 2 to 6 chips exposed.
Rather than try to measure the depth of cut, set the diamond on the lowest point in the stone and set the stop against the cutter handle at that point, thus indexing the stop to the low point. Then you can cut away the stone that is too high.