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

What is your Amp's DF and how long is the cable required?

The #0 awg is the cable for a narrow combinations of Amp's / Length.
In some cases. less is required, in some more!

For the purpose of damping the membrane speaker cable doesn't matter, since it is a fraction of an ohm in series to many ohms of the speaker impedance (breaking back EMF current circuit).  Best effective DF you can get is about 1.5 and DF>25 (not to make it much worse) is OK. Benchmark calculated that for the purpose of audible voltage drop, assuming load change from 3 to 18 ohms, DF=250 is sufficient.  The problem here is, that you might not want amplifier to do that (forcing current at impedance dips).  That's why perhaps many like low DF tube amps.

Mr. kijanki

It is interesting that you contradict yourself in the same sentence.
If you like low DF ube sound, you are welcome.
If you look into a higher DF of SS A/B class or very high DF of D class, it matters.
It is about controlling the speaker's membran (EFM).
The analysis, is going from the speaker point of view of the Amp's output stage, so it (the speaker) is not part of it. It never was. The fact we all cal the cables, Speaker cables is misleading.
Actually, the Amp's makers should have provide their recommendations.
There is a lot of ignorance in that area. The cable makers took it to a whole wrong direction, as they have no idea about their products and the # AWG significe.
 

The kenjit of speaker cables rears his head again, oh joy.

^^^^^^^^    +1