help to calculate atan2 properly

atan2 returns results in [-180,180] (or -pi, pi in radians). To get results from 0,360 use:

float radians = atan2(dy, dx);
if (radians < 0) {
    radians += M_PI*2.0f;
}

It should be noted that it is typical to express rotations in [-pi,pi] and thusly you can just use the result of atan2 without worrying about the sign.

Remove the fabs call and simply make it:

CGFloat rads = atan2(dy, dx);