Crossovers roll a speaker off above or below a given frequency. The knee in the response is where this rolloff begins and the -3db point is where the crossover has knocked down the output of the driver by 3db, which also happens to be the half power point for electrical power.
Many designs strive for the higher freqency driver to be 3db down at the bottom of it's passband, where the lower freqency driver is 3db down at the top of its passband. In effect, the higher freqency driver is handing off lower frequencies to the low frequency driver and the higher freqeqencies are routed to the high freqency driver.
Ultimately, IMHO, you are best setting these things by ear as the slope of the crossovers, driver artifacts, amplifier impedance, room gain and many other factors make it very hard to set crossover points by the math.
Many designs strive for the higher freqency driver to be 3db down at the bottom of it's passband, where the lower freqency driver is 3db down at the top of its passband. In effect, the higher freqency driver is handing off lower frequencies to the low frequency driver and the higher freqeqencies are routed to the high freqency driver.
Ultimately, IMHO, you are best setting these things by ear as the slope of the crossovers, driver artifacts, amplifier impedance, room gain and many other factors make it very hard to set crossover points by the math.