Laser beam Dispersion equation:
θ = 1.22 L/RL
where:
θ = beam divergence angle (radians)
L = wavelength of laser beam (m, see table here: http://www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Laser_Cannon--Equations)
RL = radius of laser lens or reflector (m)
These equations at projectrho also allow for realistic calculations for future military weapons, incl. Laser Cannon, Rail Guns as well as other, more far-future weapons.