Ballistic Coefficient is a product of a bullet’s entire shape, not just the ogive. Some of the features are difficult, if not impossible, to measure, so BCs are often estimated based on known BCs for other bullets with similar form factors. To obtain a true BC, bullets are fired over a given distance between chronographs set far apart to measure the time of flight. Or, more modern methods, such as using Doppler radar to measure a bullet’s performance throughout it’s entire flight. But regardless, real BC can only be measured by actual firing tests.
For most practical applications, such as generating a ballistic table, estimates of BC are sufficient. I would suppose that rockets to the moon or Mars also use estimated BCs, plus a lot of head-scratching. ;-)
One reason that BC is such an important number in ballistics is that two bullets or two projectiles having the same BC, fired at the same velocity, will have the same ballistic characteristics regardless of weight or diameter. That’s one of Mother Nature’s rules and it’s impossible to bend or break it.