No one actually knows how to lculate what speaker cable they need

For the gut with the Bryston 4BSST2 and 24’ long cables:
You are not doing well. The Bryston 4BSST2 http://www.bryston.com/PDF/Manuals/300006[3B4B].pdf have a DF of 500. You need a 3x0 AWG! it may be expensive...
You better get closer, as 8’ and use a single 0 AWG. An other option, is to used two monoblocks and a shoer (3’) of a 4 AWG cable.

For the guy with the  Opera Consonance Cyber 800, even thou you have monoblks, because it is tube (very bad DF) and no data on DF! you may use any cable over 24 AWG thick (home phone line).

Mr. sleepwalker65 your math is wrong. The cable is fighting the power (P) or the current (I). I used Klipsh speakers (8 ohms) with 99dB/w/m SPL with a Caver 250W amp. that played at
0.001W-1W as loud as it should and still a thicker cable improved sound! 
The coin been lost under the Damping Factor (DF) parameter. The higher that is, the thicker the cable you need. It is about controling the speaker, not doing a welding, towing a truck or any other idea you were throwing as you thought it is funny.  

Mr. cakyol
All cooper for electrical use is 99.5% pure. Try to get one with less!
Most cables are using wires as thick as 12-14 AWG because that’s the thicker they can fit into their banana plug (Not a very sintific one).
All qualities they say they give the cables: Cooper purity, use of silver, cryogenic treatment, Bi Wire, Burn In etc’ - has nothing to do with the spec. of a speaker cable. It’s all snake oil mambo jumbo, to cover their and customers ignorance.

Mr.  sleepwalker65
The answer is with the Damping Factor (DF) of your amplifier. There are some exceptions, if he speakers are not coil loaded (ribbon, electrostatic etc'.) or if the Amp. is a class-D type. But most, use a A-B class amp with ordinary speakers.
DF is defined as the ratio between the Amp's. output resistance (Ro, in Ohms) vs. an 8 ohms speaker, @ 1kHz.
As so, an Amp. with a DF of 500 has a Ro of 8/500=0.016 ohms.
The DF is given and you get it as you buy your Amp. It is used to be the higher the DF, the better is the Amp. Some use very stiff feedback to achive it - Not so good.
Others, may have tube amps. that have very low DF as low as 8! 
The DF is actually the way the Amp. is getting control over the speaker's coil. A coil, especially a moving one in a magnetic field, generates an electric current that is equal to the one made it move, but in the opposite direction. It is called the "Lentz" law.
Speaker cables are in a way two resistors (Rc) that connect the power Amp. to the speaker. Why two? becuse the cables go to the speaker (red) and goes back (black) to the amplifier. 
As so, the speaker's impedace has no significance in this electrical circuit.
The speaker cables resistance, just need to be significantly lower than the DF.
Other wise, the DF would decrease. So if the Rc = DF you actually have a 1/2 of the DF you paid for. This guy with his  Bryston 4BSST2, with a DF of 500 is now enjoying an Amplifier equals to a poor reciever of the 70's with a DF of about 40!
In other words, he is maybe listening to less than 10% of his equipment's full potential.
The long (24') that would count as 2xRc, is actually equals to the resistance of a 48' long cable. A few more feet and you need transportation to go visit them! :-) Why so long?
A thick cable (4 AWG and thicker) is hard to make and would cost a lot. Rather than go that long with cables, it is better to get the amplifier closer, maybe use two monoblocks and guid a blanced interconnect (XLR) that can run up to 100m with no sound degradation, while cost's little.