There are two general rules:
1: Assume that you are fighting yourself(with a large margin for errors...).
2: 5 seconds is the smallest unit of time.
So your long range point defences get one shot for every 5 seconds that missiles stay within their range without hitting anything, so you want at least as much range as your missiles can cover in distance over 5 seconds, and every additional 5 seconds reload cycle is great, and partial amounts are not useless because you don't know precisely how far away the missile will end up on any given turn... A.M.M. cannot, as a rule, shoot further than your ships can target enemy missiles, and the enemy missiles are generally moving towards your launchers...
You might want your large, cheap P.D.C.s to protect a large portion of a system, in which case figure out how far they will get from whatever they are protecting and give them some extreme range multiple stage missiles to lazily drift over the great black and then take of their coats for partytime. But otherwise the rules for P.D.C.s are not much different from ships, the enemy's evasive options haven't changed any as far as the missiles themselves are concerned. Adding a handful of longer-range missiles for pursuing a fleeing opponent is not a terrible idea, in which case assume that they move as swiftly as your own vessels...
F.A.C.s and fighters depend on the role. Missile defence and standard-missile fighters just need to travel with the fleet for long enough to do their business, which can be time enough to launch and return for reloading, move to range, launch, then return for reloading, Escort your fleet for the time it would take your fleet to expend all its missiles and their travel time...
Close combat versions though are going to be similar to missiles. If they take part in the missile exchange then they are completely missiles and need to get in, engage, and have enough to limp back into a hangar, which may be retreating for all it's worth... all at the ranges that missiles operate at.
Or you may even want completely independent fighter/F.A.C. forces and hope that your fleet can stay hidden. At which point we are dealing with extreme-range missiles again.
Also consider that one ship moves and then the other does. This can be a pretty big deal for F.A.C.s. It is kind of nice to have at least twice as much range as the distance that your F.A.C. can travel in 5 seconds so as to account for the possibilities of ships moving away from one another.
There are some dangerous things out there that do not salvage.
There are some ancient things that have long been abandoned.
There are some humble misfortunes that can be exploited before their true owners take action.
There is a good chance that "something" is there and is probably hostile. I would suggest sending something very cheap and something that can haul the wreck somewhere safer while keeping in mind that you may be wanting a fleet in the region to recover those goodies from dark forces.