Here is something else I found:
http://www2.hawaii.edu/~craiga/kktacman.html<snip>
ECM Shift (Seeking Weapons)
Electronic Warfare affects seeking weapons (i.e. drones, plasma torpedoes, and suicide shuttles) through a possible reduction to the damage caused by the weapon. It can be assumed that the ECM shift caused the seeking weapon to detonate some distance from the ship, rather than achieving direct contact with the ship. The ECM shift is calculated in the same manner as direct-fire weapons, and then the equivalent of a six side die roll is made within the software to determine if the shift has affected the seeking weapon.
Die Roll
Effect on Seeking Weapon
1-6 100% Warhead
7-8 50% Warhead
9-10 25% Warhead
As can be seen from the tables above, and an examination of the Weapon Damage Tables provided on the Ship List foldout included with your game, ECM has less of an effect (statistically) on the damage caused by a seeking weapon than it does direct-fire weapons. In most cases, an ECM shift is unlikely to have any effect at all on a seeking weapon. With an ECM shift of 1, for example, only a result of "7" (6, plus 1 for the shift) would result in a reduction of damage. If that reduction occurs it is a 50% reduction in warhead strength, which is a significant penalty. Plasma torpedoes have 3 built-in points of ECCM to combat the effects of ECM.