I just performed a test run, reloading ten times against an incoming craft. (This whole time I've been playing a game up to this point.) I guess I remembered the values incorrectly, unless this version actually uses different damage calculations/values, which is unlikely as it's the GOG version which should be, by all number calculations, identical to the original.
With plasma defences, it took either 3 or 4 hits to destroy the craft, usually 4:
4 3 4 3 4 4 4 3 4 4
If the damage is 50-100% of its defence value (900) (using air combat profile), that would average the damage to 675 and we would expect an average number of hits to destroy the craft (3000 HP) at 4.4, with a minimum of 4 and a maximum of 7--which is a lot closer to what I had remembered from before, anyway.
If the damage is 50-150% as Warboy said, that would average the damage to 900, with average hits to destroy it being 3.33, max hits 7, and minimum hits 3. That is a close match to the data I gathered.
I guess I was wrong. Thanks!