Limiters for speaker protection "limit" the amount of voltage an amplifier can deliver to a speaker. This should go a long way to protecting the speaker from thermal burnout (you still need to protect against excursion limits with HP filters). It should be said for the record that it doesn't matter what the waveform looks like ... only the amount of power contained in it ... square wave, sine wave or anything in between. Amp clipping itself does not burn speakers and underpowering does not burn speakers. Too much voltage for too long will burn the voice coil or separate it from the former itself. Protecting your amp from clipping has very little to do with anything except your amp (and most amps have clip limiters built in)
Let's start with the basics and add exceptions as we go along
Set the limiter for about a 20:1 ratio or higher. In the real world there is almost no practical difference between 20:1 and infinity:1 (because you have almost nothing to drive above threshold. If you set the threshold at "0" you'd have to drive 20 dB more to get the output to rise 1 dB). Anything less than 20:1 doesn't give you much protection. The attack and release will depend of the averaging style of your limiter and the frequencies involved. You probably want the attack on the order of 15ms - 20ms for woofers and maybe 5ms for tweeters (not written in stone). The release is generally set to be 10 times the attack time.
Now comes the tricky part ... how do you set the threshold?
Now you need to know how much voltage your speaker will handle. It will be called the "continuous", "average", or "rms" rating. If it is called "program" then cut that number in half and if "peak" divide by 4 (generally). Understanding the "real" number is a bit difficult ... so setting a limiter like this depends on how good this number really is. Adjust (down) as necessary for your own comfort.
It is helpful to know by which method the manufacturer has used to make this rating. If done by the AES method then it considers the minimum impedance. Speakers have a "nominal" rated impedance e.g. 4 ohms or 8 ohms but the true impedance varies with frequency. It would be typical for an 8 ohm speaker to vary between a low of maybe 6 ohms (at low frequencies) to a high of 20 or 40 ohms at high frequencies. So your amp is delivering varying amounts of "wattage" at different frequencies. You may have to subtract a little bit of power handling ability.
Convert the speaker rating to Volts;
sq root (speaker rating * impedance)
so a 500W 8 ohm speaker looks like this ...
square root (500*8) = 63.24 volts
(just type "square root (500*8) into google and it will do the math for you)
At this point you can simply drive signal through your limiter to your amp and bring down the threshold until you reduce the output of the amp to 63Vac measured on an AC voltmeter (assuming you have proper gain settings)