IIRC, it works on the basis of percentages. If you increase case capacity, one fourth of the powder percentage increase will be the velocity percentage increase. For example, if you increase the case capacity 4%, you increase the velocity by 1%, given that you are using a powder that produces the most optimal velocity, and you are using the same operating pressure. If you increase the powder capacity by 10%, you increase the velocity by about 2.5%, again using the most optimal powder at the same pressure.