What ratio of power in tri-amp system ?

In order to better utilize equipment , is there a formula or chart somewhere that recommends what wattage power amp should be used for each power amp in a three way system ? I realize that crossover points and speaker component's individual performance spec's would cause great variation , but is there some kind of rule of thumb for a starting point ? Is this too vague a question ? I have a PA system with separate cabinets for all speakers and would like to know whether I have enough power for the speakers or need to get more speakers for the power .