It should be possible to expand the table to cover in-between cases. I did something similar for designing my own take on Star Wars style blasters. Here's a modified version of that progression:
Code:
Output dDam
100 kJ 1d
150 kJ 1d+1
200 kJ 1d+1
300 kJ 1d+2
500 kJ 1d+2
700 kJ 2d-1
1 MJ 2d
1.5 MJ 2d+1
2 MJ 2d+2
3 MJ 3d
5 MJ 3d+2
7 MJ 4d+1
10 MJ 5d
15 MJ 5d+2
20 MJ 6d+1
30 MJ 7d
50 MJ 8d
70 MJ 9d
100 MJ 10d
150 MJ 11d+2
200 MJ 13d+1
300 MJ 15d
500 MJ 16d+2
700 MJ 18d+1
1 GJ 20d
Note this is slightly different from the default SS progression - mine goes 3-5-7 rather than 3-4-6 - in order to use the SSR progression from the rest of GURPS. This only makes a difference at the 10 xJ and 30 xJ (where x stands for any prefix) points.
This is only dDam1. For dDam2, just multiply output by 10 to determine damage.