For Bass, set it by ear. In my unusual system where the custom subwoofers are good to 350 or so and the mains are MG1.6, good to 40 Hz, the best setting depends on the music. I have a convenient way to change the frequency (a knob).
What do you consider when setting crossover freqs?
After all I have read, I am still unclear as to whether the cross over for a sub is set at the frequency where the mains/fronts taper off (i.e. the non-flat portion of the frequency response curve) or it is set up where the spec say the mains/fronts are responsive to, say down to 60Hz). Or none of the above.. or other factors I do not know about.