Good firing clay and good
cob clay are at least mostly the same thing.
Structural cob can be something like one part clay to two to four parts sand, plus enough straw to make the mix hold together. I tend to sprinkle about as much volume of loose straw as there is clay and work it in, for a very rough rule of thumb. Do it in several stages so you don't end up with clumps of straw packed together.
The clay/sand ratio doesn't account for how much sand is already in the natural clay deposit. Some may be relatively pure clay that needs a lot of sand added; my clay bank has so much sand, silt, gravel and rock in it that I can make good cob without any additives at all.