Beam divergence and diameter

In optics, laser beam divergence refers to how much a beam spreads over a distance. Imagine a cone where the tip is the theoretical origin of the laser beam (where the beam diameter would be punctual). If you measure the angle between two vectors, one going from the tip of the cone to the center of the base and the second going from the tip of the cone to the side of the base, the resulting angle is what you would call the divergence half-angle. Obviously, the full angle would be double that. Then, knowing three out of four parameters amongst initial diameter, final diameter, distance in between diameters and divergence angle, will give all the necessary information to calculate the fourth parameter. Also, if one understands the picture clearly, he will realize that the initial diameter can be, in fact, any diameter along the Z axis and doesn't need to be the one at the laser's exit.