Equal-width miter joints
Omni's miter angle page starts with the simplest case: when both boards have the same width, each miter angle is just half of the joint angle. A 120 degree corner therefore needs two 60 degree cuts.
Different-width formulas
Equal widths: miter angle = theta / 2
At 90 deg: board A = arctan(a / b), board B = arctan(b / a)
Narrower board = arctan(n x sin(theta) / (m + n x cos(theta)))
Wider board = theta - narrower board angle
Miter saw angle = |90 deg - miter angle|
Saw setting versus cut angle
The fetched source clearly distinguishes the miter angle from the miter saw setting. The saw is set to the complementary angle, which is why a 79.11 degree cut uses a saw setting of 10.89 degrees.