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. 

I'm very happy with them.

I got 2x1m and 1x2m cables, each with 8 silver solid core wire, cryo'd, with built in jumpers at the speaker ends, solid sliver spades, and made with silver solder. Each cable is in a teflon sleeve, then litz braided, then covered in an outer sleeve. Additionally I had mine covered in nylon braid, just for aesthetics.
I also got 5x1m XLR cables. Total cost was £540.

I'm was going to get some clearday and cabledyne cables as well, to compare, but clearday aren't making any more and cabledyne aren't in stock.

As for sound, they're very nice. Not too much added brightness, a little more clarity and transparency, and better instrument placement. I think I'm limited by my physical space now, rather than any component or cable.

So far I've tried: van damme blue (not highly rated), 0 gauge battery cable (a step up from van damme, but that might just be a cable maintenance issue), kimber 12TC (nice, but a nightmare to work with).

I've got to a point where I'm probably going to stop looking at cables for this system until I move. Once I have a larger room with better dimensions I'll come back to them. At the moment I feel I've got about the right level of price vs performance that I can get in my current space.
Thanks for your input, I think I am going to give silver a try, I only need a 5ft. pair as I have limited space too...

   what type of conductor do you use? Btw, I have known & done this for decades but I still haven`t needed to use 0awg on anything. 

cakyol; you are correct on one thing but wrong on an other
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.