Cartridge Base to Ogive (CBTO) is the measurement from the base of a loaded round to a specific point on the ogive (nose) of a bullet. When loading at home, you generally use a set of calipers and an ogive comparator tool to find what the CBTO would be.
One common issue with measuring this at home is the fact that even with today’s advanced machining techniques, two cartridge comparator tools for the same caliber are not completely identical, so there is almost always variance between two different sets of tools. There is also the fact that calipers are not a reliable tool for measuring down to thousands of an inch due to even the most capable operators being prone to having variance in measurements. For a more accurate physical measurement of variance in CBTO, you should use a force push gauge to put the bullet into your comparator, then use a drop gauge instead of calipers.
On the industrial side, newer Quality Assurance tools with optical lasers can measure down to the micron and can simply measure in a reliable way that you could never get with hand tools, which are prone to human error. These are the kinds of tools we use to measure here at Berger.