Thats maybe the wrong approach in thinking about it.
3 armor would reduce the enemy chance to kill the missile to 25%, yes, thats less base bonus for the missile, but instead of two, on average he needs 4 shots not to kill that missile.
Which while only double as much than 2, is still two more, so for every point of armor, the enemy needs one more shot with classical point defense weapons.
Sure, 1 point is mostly enough anyways, but still, armor doesn't really get inefficient if you apply more.
The missile, however, does.
For example: A size 6 missile has 2 sizes allocated to WH, being 10 (why anyone would have 10 WH is beyond me, but you never know).
Reducing that by one (5) allows for one point of armor.
If you always penetrate the enemies point defense (or at least more that 50% of the time), armor will probably be detrimental to the results.
If the enemy easily beats you back, you won't have an effect.
If, however, the enemy can barely beat back the great majority of your missiles, for easy of example 100%, and doesn't have time to fire again, one point of armor will make 50% of the missiles survive, thus greatly increasing the results.
As a matter of note, Armor is better than ECm at start, but becomes worse once you get some serious ecm tech.
If the enemy only has a low ecm level, say, 4 levels lower than you, which is indeed a technical advantage, you can reduce the enemies hitrate/missile range by roughly would your armor would do versus point defence, while also working versus stronger weapons.
An ECM advantage, and be it just 20%, will work fine versus mesons, Laser, allround Missiles, and might end up denying the enemy an extra salvo of Anti-Missiles.
Armor on Missiles does never improve with tech.
