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

Amp DF = 50
4 ohm speaker
6 ft length

Wire gauge?

Mr.  eichlerera

For DF=50 and length of 6 feet (2 m), you need a #16 AWG.

1. No need for speakers impedance. They are not in this game (speaker cable).
2. DF=5 is relatively low. It was common (40~) at Japanese amps and receivers of the 70's. 

Well, since my Fidelium Cable is between 14 and 16 AWG, looks like it passes your criteria. Plus it's skin effect issues are minimized.

Mr.  eichlerera

I'm sorry to kill your impression of skin effect on Audio frequencies.
The figure (at the AWG table) refers to full power at x frequency.
An other way to look at that, is that thin cables have low current capacity at DC, so it is close or identical to that in all Fr. Thick cables, that can conduct 10th or hundreds of Amps at DC, are a bit less of higher Fr.
But even so, if you look into Audio energy vs. Fr., the most power is at the lower Fr.: under 1 kHz. most of the stuff over 10 kHz are harmonics that are of very low energy. So skin effect is not for Audio, It mostly present at RF and very high frequencies, at high power (Radar etc'.).

b4icu,

How do you believe the signal propagates through a speaker cable?