For your missile sensors (they can be passive as well) you want them able to detect any ship in the target task group at the point that you original target is no longer valid. In general I take this to mean detecting a target within 5-10 seconds of flight time, or 1 cycle of your standard launchers, whichever is greater. The 5-10 second will give you a chance to get some use out of salvo's arriving after the one that destroyed your target. In this case they will split their fire between all valid targets (resolution being a factor for active sensors) This is most usefull when you are throwing lots of small salvo's at a target, ie fighter or swarm style tactics. For the cycle time of you launchers this is to allow for sending to many salvo's at a target over time. Just a quick example as to how far a ship might travel in 30 seconds think about a 4000km/s ship. In thirty seconds it will have gone 120,000km. If you only detect something out to 30-60,000km them you wouldn't even see them.
As for enemy ecm it will affect your onboard launchers, but not the missiles self guidance. Also you can put ecm on your missiles, but not eccm. If you have a significant ecm advantage over an oponent this can be good as 1 missile space point (msp) will give you the max ecm you have researched. This can make a big difference on the counter launch range, and especially for their beam point defense. This is however a tactic for large missiles not small missiles.
Good Luck on your next missile combat
Brian
