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

Folks, everyone is talking resistance but there are two other factors that come into play.   One is the inductance and capacitance of the cable, the other is the skin effect.   For audio work, skin effect becomes a non issue when the wire is 16 g copper or thicker, and I doubt anyone reading this is using 20 or 18 g wire.  The other is capacitance and inductance and these are tied together.  If you are using dynamic loudspeakers, that is a speaker whose motor is a magnet with a voice coil and cone, then one needs very low inductance cable with the result of it being higher capacitance.  The reason is inductance becomes a significant factor when the load impedance is low and most speakers have impedance dips below 8 Ohms and peaks above 8 Ohms.   This causes an attenuation of signal if the cable is not low inductance.   Plain zip cord is fairly inductive and thicker zip cord, that is the stuff with larger gauge copper, becomes even more inductive while the resistance drops.   This is the reason speaker cables sound different.   If you have a non dynamic speaker, that is a pure resistance load, i.e. Magnapan, ribbons, etc. or an electrostatic, then the parameters change.   Resistive speakers require low resistance and a moderate inductance cable while electrostatic speakers are a special case but can more or less live with higher inductance cables.  There are other losses as well, but these variables are secondary to the above.
Regarding the question why no manufacturer will calculate the cable needed is a lot depends on the output impedance and phase margin of the amplifier.   One just can't hang high capacitance on the output of an amp, it might cause it to oscillate. At the very least, it won't like the load; worst case is the amplifier dies a violent death.  No cable manufacturer is going to take that risk, rather it is left up to the user to sort all this out.

Plain zip cord is fairly inductive and thicker zip cord, that is the stuff with larger gauge copper, becomes even more inductive while the resistance drops.

Inductance of straight wire is lower for thicker wires.  For instance gauge 16 wire has inductance of 0.33uH/ft  while gauge 10 wire inductance is 0.29uH/ft

Mr. spatialking

You are a late comer to this thread. The issue was brought up earlier.

1.       The DF and the speaker 8 Ohms values are ref. as resistive only (https://en.wikipedia.org/wiki/Damping_factor):

In an audio system, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive.

2.       This is not the reason speaker cables sound different!

There are as many attempts to explain, justify, point at factors without really checking it or study the matter. Impedance (as it is a complex "resistance" that includes an inductive (coil) and capacitive values, is none relevant when a speaker's cable (a good one) is way under 0.002 Ohms. For a cable, no matter how bad it is made (as a copper wire alone shall have no inductive or capacitance at all!) the values of those inductive (coil) and capacitive values are so low, that referred to the resistance value they have no effect.

Those values, with resistors of kilo Ohms do. This is a ratio of 1:1,000,000! Between 0.002 Ohms and 2k Ohms.

 

b4icu,
I am unable to fill in the excel table with my iphone. I can view it with the link you provided but unable to update with my input as I may not have the app. Please help

as a copper wire alone shall have no inductive or capacitance at all!

Of course straight wire has inductance. It is on average in order of 0.4uH/ft that represents about 0.2ohm at 10kHz for 8' wire.