For example:

The 8mm Mauser SAAMI specs for the brass just above the extractor groove is .4716". That is what the diameter of new, sized brass should be. When you fire, you measure the same point above the extractor groove on the case. If the measurement is more than .4721" diameter, youare over max for the gun. Then itsays to drop down the load one full grain, and you are at max.