Well, I just looked at it myself, and it looks like damage is, roughly, 3.5 points per D. So for instance, a Laser Rifle, damage 5D, looks to be 3x + 6. 3x is essentially 3D, and the 6 is just about another 2D.
The next part is deciding when to use x1 (pistols) versus x2 (rifles) versus x3 (plasma weapons, and laser wepons, except for the pistol), versus x4 (fusion weapons).