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

Simple math. Take into account the power demands of the speakers to achieve the desired SPL. Then the power output of your amplifier, and subtract the two. This gives you the margin that can be lost to resistance in the speaker cable, P=I^2*R. You know the power needed to drive your speakers, and you know the speaker average impedance, which gives you the amount of current by plugging in the power and approximating resistance with average impedance. Take the power margin from above, and thus current value and use the same equation to solve for resistance of the cable. Divide this resistance by the distance in feet to obtain maximum ohms per foot of cable, and go shopping!

ff

Opera Consonance  Cyber 800 monoblocks to Focal 936 Electras connected with Jena lab Trio speaker cables A really nice pure all copper cable.

For the gut with the Bryston 4BSST2 and 24’ long cables:
You are not doing well. The Bryston 4BSST2 http://www.bryston.com/PDF/Manuals/300006[3B4B].pdf have a DF of 500. You need a 3x0 AWG! it may be expensive...
You better get closer, as 8’ and use a single 0 AWG. An other option, is to used two monoblocks and a shoer (3’) of a 4 AWG cable.

For the guy with the  Opera Consonance Cyber 800, even thou you have monoblks, because it is tube (very bad DF) and no data on DF! you may use any cable over 24 AWG thick (home phone line).