It makes it up as it goes!
So I was testing out some PD weapons to make sure they were reasonably balanced and tried to work out how exactly point defense damage works.
I failed miserably. All I managed to work out is that it will always take at least two hits to destroy a missile and that mass damage seems to be twice as effective as energy damage. Here are the results of my testing against a missile with 10 durability.
energy damage shots required
5 6
6-7 5
8-9 4
10-19 3
20+ 2
Can anyone work out some sort of formula to fit those numbers? I had a theory that the first shot does no damage.