calculation are made in radiant and translated to degree for easy reading
sin(x) -> x as x -> 0.
That approximation is the better when x is smaller.
i can understand drift and other approximation/errors give you to wrong value, but i still cant' understand where this "90/256 ~= 0.4" come from, in particular the "90"
In 90 degrees, the sensed gravity on the z axis goes from 1g to 0.