Stoppers to measure OAL with a cleaning rod

Obviously we cannot see or be absolutely sure that the case of the MAMBA OAL gauge was seated fully and deep enough into the chamber. We MUST therefore check the measurement of the OAL gauge by using the rod method;
The well-known method of making marks on the rod does not work so well. How square do you make the marks, how wide are the marks and how consistent are you in measuring exactly to the mark?
MAMBA provides a bush that is attached to the rod with a clamp-screw to ensure more accurate and repeatable measurements.