The “book” loads are determined by using test barrels and strain gauges. They don’t load up until they see pressure signs and then back down to determine max. They fire hundreds of rounds and actually measure the pressures. SAAMI has determined the max pressure, so when they reach that pressure, that is the max.
That being said, each barrel and chamber are different, which is why every book recommends backing down by 10% and working up slowly.
