There is no formula and if you don't have the proper analysis tools and the expertise to use them then you may be limited to picking some starting
point and playing around until you find what sounds best.
A major reason for this is that while the electronic
crossover affects the audio signal, what you actually hear is the acoustic
crossover between the outputs of the subs and mains. Thus the relative levels of the subs and mains, their response through
crossover, their interaction with the room and even the relative locations of the subs and mains to one another and to the listener or measurement
point can also factor into what are the 'best'
crossover settings.
Another factor in this is the
crossover itself and what adjustments it allows. It may be a simple
crossover that only allows selecting a frequency. Or it may be a more advanced device that allows adjusting
level, corner frequency, filter type, order/slope and even delay independently for the high pass and low pass filters. And for subwoofers, along with the
crossover to the main
speaker there may also be a high pass filter to filter out frequencies that are below that sub's range.
If you do have to guess at a starting frequency that may depend upon the boxes involved. For a full range box with a 15"
woofer and significant low frequency output used with an 18"
subwoofer you might start at 80Hz or so while for a 15" or 12" sub to a very small main you might want to start at 120Hz or even 150Hz. The more information you can provide on the mains and subs involved, the better estimate at an approrpiate starting
point that can be offered.
