EHD causes an an additional hull hit no matter what the damage roll is. Essentially you're guaranteed to have a number of hull hits equal to the DMG dice thrown plus the result of those dice (50% chance of another hull hit). So, on average, Hull Hits = DMG Dice * (0.5 + 1):

#DMG Die    Hull Hits
   1          1.5
   2          3
   3          4.5
   4          6
   5          7.5

Catastrophic DMG dice have to come up odd (a hull hit) in order to trigger the special ability. Once that happens you're rolling a d6, where the average value is 3.5. So, on average, Hull Hits = DMG Dice * 0.5 * 3.5:

#DMG Die    Hull Hits
   1          1.75
   2          3.5
   3          5.25
   4          7
   5          8.75

Then, just for comparison sake, here's what it looks like with neither:

#DMG Die    Hull Hits
   1          0.5
   2          1
   3          1.5
   4          2
   5          2.5

EHD is 3 times better than normal, so a mod of 3.0. Catastrophic is about 17% better than EHD. So, 3.0 * 1.17 = 3.5, the catastrophic modifier.