1. Where is this "formula"? I went back through two weeks of posts and still could find no formula, just multiple comments about basing the wire size on the amplifier and how "0 gauge" seems to usually work well, and also a couple of comments about needing thicker than 0 awg in cases where the SCs are longer. When I started to see other posts asking "where is the formula" I assumed going back further would be of no help. I would like to apply this formula to my situation and calculate what gauge wire I would need to meet your formula based on my SS amplifier.
2. I read your opinion about separating wires and assume your DIY results in running separate pos and neg wires from the amplifier to the speaker. Your point seems to be that this should resolve mutual inductance and that resistance (i.e., inverse of wire size) is the governing factor. Some manufacturers have done this (separated pos and neg wires) in the past but typically that approach has not seen long-term commercial success (low WAF?).
One way to get pretty close to your 0 awg would be to purchase 8/3 50A NEMA welding extension cord and then connect the three internal wires together and use a single run separately to each pos and neg terminal. Each post would then see just under 3 awg (i.e., 3 x 8 awg ~ 3 awg)
To actually achieve 0 awg wire, I would probably purchase 0 awg battery jumper cable wire in the length I need and crimp a short, large gauge, lead-out wire to the jumper cable wire that would attach to my speakers and amp. I would run the two battery jumper cables separately with one going to each of the pos and neg posts.
Where is the formula?