Here is a revised damage chart for the Plasma X. The OP manual is not correct. There was some variance on damage at the bracket ends, but I would chalk that up to the model and the placement of the Plasma launchers on the ship (what I mean by this is that I would fire two different Plasma X's from the same range and get two different damage values, but it was not consistant). As you can see the range breaks are a little different as Plasma X degrades faster.
Range:
0 to 8.25 = 40 damage
8.26 to 12.15 = 29 damage
12.16 to 16.15 = 20 damage
16.16 to 20.15 = 13 damage
20.16 to 23.30 = 6 damage
23.31 to 24.10 = 1 damage