If I understand it correctly, the current formula for PD working on a missile with decoys is:
Chance to hit missile = 1 / 1 + 0.2*(# alive decoys)*clamp(Missile ECM - PD ECCM + 5, 0, 5).
In effect, it means that if you aren't confident your ECCM is better than their ECM, you might as well strip ECCM from your PD dedicated BFCs (especially point blank rail/gauss), since the decoys will operate at full efficiency regardless.
So far my testing confirms the upper clamp, that is, a decoy cannot be MORE effective than "the same as a missile," and the lower clamp is fairly obvious.
If that's correct, I find it a little weird. Not difficult by any means, but yet another thing that's a little unintuitive, that i'll have to update all my designs for. If I could tweak the numbers myself I would, but I'm totally unfamiliar with C# decompiling and tweaking so that's a no-go unless there's relevant numbers in the DB... which I doubt, but I'll see soon.
There's two "simple" changes that can be made to make this behave more intuitively. Either, or a combination of both, could be used. Or neither, if this little quirk is desired behavior.
1. Allowing decoys to be bigger than a missile, aka removing the clamp on the upper end. Maybe just swapping clamp 0,x,5 for max 0,x. Maybe making it so that it scales slower when it's "bigger", so maybe 2 ECM vs ECCM advantage is required to make the weight go from 5 to 6, so to speak, then another 2 to go to 7, or maybe another 3 to go to 7. All subject to tuning.
Under this regime, ECCM is a losing game but not playing it is even worse, and by a lot. Not putting ECCM basically makes the PD BFC useless, and turns amm into a decoy sweeper.
2. Shifting the center of the linear section closer towards equal ECM/ECCM. AKA, keeping the clamp 0,5 but changing the base to 3 or 4, so having 1-2 level higher missile ECM gives max bonus and having even ECCM gives tangible (but small) benefit compared to running 0 ECCM.
Under this regime, Missile decoys are worse to begin with, but ECCM now can actually be significant at equal tech (20% reduction if the timings happen to line up well for you and a much higher chance of doing nothing is not "significant" in my eyes). You CAN choose to give up the electronic war if you know you're VERY behind, but if you think you're competitive you can try to fight on this front as well.
For example at C = 3, if you think you're equal tech but actually you're 1 behind, you still reduce decoy effectiveness by 20%. If you ARE equal tech, you reduce decoy effectiveness by 40%. If you're AHEAD, you get even better results. As the missile party, you still get some effectiveness from decoys even at ECM 1, but ECM 2 and 3 now get additional importance as "sealing the deal" vs 0 ECCM skimpers.
Having said all this, once again I stress: if this design quirk is intended behavior, then feel free to ignore this post. I'll look for solutions on my own, because I'm a dials/knobs addict, I can't stop poking at them until it feels just right, and giving up entirely on the electronic war entirely just because your tech isn't strictly better feels VERY wrong to me.
EDIT: Also, ECCM is double RP cost compared to missile ECM. If a species is teching hard into missiles, they will very likely be on par or at most 1 tech (1/4 RP cost) behind on ECM tech. I don't need to point out how this is especially brutal late game or low research, where it basically means you can't really "win" even on the tech side before BFC manufacturing cost comes into play.