How is gl_FragCoord calculated. ?

I need to shift the depth of a fragment a little bit (converting circles to spheres). Can anyone explain me how the original gl_FragCoord.z value is calculatged ?

I’m sure there are lots of places, but here are a few:

here is another OpenGL.org thread:
http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=255766#Post255766