You are dealing with 2 curves and the idea is to blend them so that the result is a flat line. Not possible but that is what you are aiming for. For example, if the main speaker is -3db at 60Hz you would set the subs crossover at a point that would boost 60hz output back to flat. Lacking sophisticated measuring equipment (which is getting cheaper all the time) the best thing to do is set it by ear, as all rooms will boost or suck out the bass to some degree. Get a CD or LP with test tones at different frequencies and set the crossovers to produce the same level of sound at the different frequencies. Some subs have complex circuitry to do this for you but you can do it for yourself, it takes longer.
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.