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

Who is "We"?  (For example we already know the cables the guy in UK)
What is your hi end brand or model? A link would be nice.

It’s the editorial “we.” We’ve already discussed the UK and his Blue cables, the ones that did not fair too well in at least one review. It was about two months ago on this thread. Of course, many posts have mysteriously disappeared since then. But I have a photographic memory, thank goodness. 

I seem to remember in one of your 220 posts, on this subject, your criticism of the cable companies not providing details on wire design & construction. Well, here you go:

http://www.iconoclastcable.com/story/speaker.pdf

Looking forward to your white paper.

Let me examine one statement made before by b4icu:

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.

 Of course it has big significance.  Speaker's resistive impedance (equal to about 2/3 of nominal impedance) is in the circuit.  The whole idea of damping comes from the fact that voltage created on the speaker terminals (known as the "Back EMF"), by the motion of the coil in magnetic field, produces current (flowing thru speaker) that causes opposite membrane motion effectively stopping the membrane.  This current flows from one speaker's terminal thru speaker wire, amplifier's output, another speaker wire, another terminal and the coil.  Changing resistance in the circuit from 0.1ohm to 0.001ohm means change in total circuit resistance from about 6.1ohm to 6.001ohm (for 8 ohm speaker) resulting in very small change in the "braking" current.  

I don't know why do they make very thick cables.  Perhaps to reduce inductance (straight wire inductance is slightly lower for thicker wires), but this would not make sense since the same can be achieved by better twisting of thinner wires (that reduces inductance and increases capacitance).    One possible reason can be skin effect, that starts at gauge 18 (20kHz, copper).  I'm not sure if it is audible, but remember Audioquest FAQ.  Stranding wires even with isolated strands won't help since wires are still in magnetic field of each other.  Placing wires in helical twist on hollow tube serves two purposes.  It subjects strands mostly to magnetic fields of neighboring wires only and provides twist.  This twist not only reduces wire inductance but also provides immunity to external electric and magnetic fields.  Since each of perhaps 20 or so strands has decent gauge (for the ease of working with) it makes overall gauge much lower.  That's how my Acoustic Zen Satori is constructed.

One observation  - amplifier's output has very low impedance for low frequencies only.   High frequency electrical noise induced in the cable is injected into the output of the amp and amplified by the amp because of negative feedback.

As for cable directionality - yes, AC charge flows forth and back but energy to speaker is delivered one direction only, on the outside of the cable where other factors (including insulation) might play role.  Again, I don't know if direction is audible or not - but it is much more complicated than it seems.

Mr. khiak

You have been ignoring my two requests:

a.     To share your impression about the sound with your new DIY cable.

b.     Fill in the data missing on the excel table.

Please do.

Your approach regarding the equation of the amp-speaker circuit, with the speaker cables in between is wrong! If you would isolate (as you did) the DF from the circuit, and use optimal cables (0.0000 Ohms), it still not the 8 or 6 ohms of the speaker impedance, vs. the DF resistance value.

DF is provided by the standard, in an 8 ohms load (no matter what is your speaker's actual impedance!) and at 1 KHz. Some brands would extend that data into higher Fr.

However 1 KHz is not a low frequency at all.

Directionality: What directionality in AC where the wave is symmetrical both ways?

Skin effect: When you look at the AWG table, you see that the data provided is regarding the top Fr. That a cable can still pass it's full power. A 4-0 gauge cable is way thicker than an audio system needs to pass through its full power. At the playing power (even at peak) the 4-0 gauge cables still pass more than sufficient at 20kHz.