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


It goes back to cable manufaturars, mostly provide no relevant data! to sales and the users. None will answer this!
Whay do you think that you own now the optimal cable to your setup?
I think I've figured it out. 


128x128b4icu
On top, it dosn't alter my opening line:
No one actually knows how to lculate what speaker cable they need

What you say, when I'll here it, I'll know it is. Till than, just going in circles, spending money and enjoy the journy. Hope you are not on the same page on other subjects, as health.
Why would I get lost over and over agin, waist my time and money, rather that use a GPS nav device or a map, and get there first time on spot?
For the un of it...
As for the optimum, as I assume you already own an amplifier and a set of speakers, they are given. so is placment that requires the length.
You may never know if your preference is right, till you get to hear the optimum (the one) and compar.
Other cables: if I use XLR's and an ordinary catle power cord, I'm good. 
It won't get any better than that.

First show me your "optimum" speaker, source, amplification, etc.

The premise of the post is fallacious. :(

Anyone with experience knows that what you are asking for is not relevant to achieving the most pleasing sound with an audio system. Go ahead, procure measurements; they will bear little relation to whether you actually prefer the sound of a particular speaker cable. YMMV


BTW, leaving other cables out of the discussion ruins the entire endeavor. You cannot properly tune an audio system by placing focus only on the speaker cables. You may as well guess at it.  :)

Thanks, but sorry...NO.
I'm talking here on speaker's cables only. So lets leave the other cables out ot this discussion. 
The only insite I have from your answer, is that
You say "try and error with a budget".
I say, "Tell me what brand and model your amp. and speakers are, what is the distance between them - and I'll calculate for you what cable is required".
The optimum cable for the money. First time - and also the last.
Any cable; interconnects, speaker, power, etc. will work in ones system. It is up to the end user to experiment & determine what cable they like best in their system and how much they want to pay for it. It really is quite simple, just treat it as you would another component.