The movement of one small hash mark is equal to 0.1 mm. And every big number in the dial represents the movement of 1 mm. Therefore, one full turn of the dial represents 10 mm.
There are numbers (0, 10, 20) engraved on the shaft. If it comes between 10 and 20, it means that the dial is reading between 10 and 20 mm. In that case, you should add 10 mm to the reading value to get the actual value.
When you measure the outer diameter of the target, rotate the bezel so the initial reading is set to 0.
The dial reads 7.2 mm. However, the shaft came between 10 and 20, so the outer diameter of this clip is 17.2 mm.
When you measure the inner diameter of the target, rotate the bezel so the initial reading is set to 3.