I see a lot of speculation on how to set your crossover points accurately so here's a quick and dirty way of doing it correctly.
First, figure out the point that you want the crossovers set at. Second, download WinISD, go to signal generator and set it to that tone, open windows sound recorder, record the tone for a few minutes, stop recording and save it.
You can burn it to CD or transfer it to a thumb drive, if you're using multiple crossover points just do the same thing for each point.
Third, play the tone, set it to repeat and bring it up to a decent level that you can recognize audibly. It doesn't have to be loud, just a few watts will do. You'll need a good DMM for this next bit or a good db meter.
Step four is to figure out your relative level using either the DMM or db meter. After doing that you adjust the crossover point until the output drops by -3db. If you're using a DMM simply got to the 12V.com's Ohm's law calculators
and plug in your starting voltage, the resistance of your speakers and find the wattage. Next go to the bottom calc and plug in half that wattage value along with the resistance of your driver to find out your -3db point voltage. Now go out and adjust the crossover to hit that value.
For example, if you've got a 4ohm speaker and it's getting 20V then it's getting 100W. The -3db point is 50W and 50W at 4ohms is right about 14.14V.
That's it. If you're using 80hz (for example) as your desired crossover point with a 12db per octave crossover then the output at 80hz will be -3db. For an 18db/octave crossover it's -4.5db and for a 24db/octave crossover it's -6db.