Here's a simple C function CWYdeg(x,y) which returns an angle from 0 to 360 degrees measured clockwise from the +Y Cartesian axis when given Cartesian coordinates (x,y).
Using single-precision floats, the accuracy is better than +/-0.17 degree over the entire 0 to 360 range.
Using a similar technique, it is straightforward to create a function which returns angles either clockwise or counterclockwise, from the +X or -X or +Y or -Y axis, in the range 0 to 360 or -180 to +180 or 0 to 2pi or -pi to pi