It is certainly not 255, or I am a no good son of a Phong.
Go test it out with absurd numbers in a test game (do not use the stupid combat simulator, as it is glitchy). Give a 1000 CS and 980 ECM bonus to some ships/components and see what results you get. If you end up with net 20% bonus to hit bonus. Also try 970, 960, 950 ECM.