Before anything else is said this rant is mostly in relation to spacecraft and unguided weapons. Guided weapons have their own partly similar but different measures for their effective range which can get pretty complicated. I might get to that at some point.

Effective range is the range where it is actually practical to shoot at target and expect to hit and/or do damage. For instance your effective range against the moon with a laser pointer would some amount of light seconds to minutes with the limiting factor being the distance where the moon gets too small to see or aim at. Whereas the theoretical maximum range would be so far away that every photon from the laser pointer has a 100% chance of being dissipated by the interstellar medium even if it was fired and perfectly pointed at the moon for the laser pointer's entire battery life. Generally on WWW range is simplified quite a bit by being brought down to a single factor (generally the longest range that the attacker has hit a target). That isn't necessarily a bad thing, if getting into the full detail of an issue stonewalls/derails the discussion then it is better to back off on how complicated you are making the problem. But sometimes that simplification is a bit of an oversimplification. The biggest simplification probably comes from ignoring that effective range depends as much on the attacker as it does the target.

Attacker

The three general things for the attacker's part of effective range are sensors/fire control, accuracy and muzzle velocity.

• Sensors/fire control

Pretty much anything that is able to hit other things has some combination of sensors and fire control. For a boxer throwing a punch it would be their eyes (sensors) brain (fire control), for a WWII battleship they might be radar (sensors) computer (fire control).

Sensors are important because you can't hit what you can't see. In my boxer example if the boxer's opponent had a bunch of 100 kW lights (and sunglasses) the boxer would have a much harder time hitting them as their primary sensor is being "jammed". There are two things you should be looking for in sensors, type (how the sensors sense things), what kind of object/thing it is detecting and range. Type is important because various settings have ECM/Stealth and if a ship has sensors that the other side doesn't have ECM for then the ECM won't work. Certain types of sensors also have caveats like passive vs active or FTL vs STL which are good to note. What kind of object/thing they are detecting is important because this plus detection range determines the fidelity of the sensor. Detecting smaller less radiant things is generally better than detecting bigger more radiant things. The reasons why sensor range should be gotten should be obvious.

Fire control in SF is generally the work of a non-shitty computer thus a non-issue and can be assumed to be capable of plotting at a minimum basic predictions of where the target is going to be. If you are picking up feats for fire control specifically it should either be be things that are unusual, like if the computer can look into the future or how well it preforms against ECM. Generally it is tough to compare ECM between settings (except in cases where it is extremely one sided like say a Culture GSV vs a modern fighter jet) but it is good to try to find feats of it burning through ECM.

• Weapon range

This is the most common part of effective range you will find on WWW. Usually it takes the from of people just listing out stated numbers for the ship hit a target at X distance. It is important to note if they are always hitting, consistently hitting, or often missing at this range. This is one of the more important parts but it isn't the whole story but we will get to the rest of it later as that relates more to the target rather than the attacker.

• Muzzle Velocity

Muzzle velocity is just the speed of whatever the attacker's weapon moves at. This is important for figuring out the projectile/beam's flight time (aka the time it takes the projectile/beam to reach the target after being fired which is how much time the target has to try and dodge) which is very important when attempting to actually apply effective range in a cross-setting battle on WWW.

• Summery

What you want from this section,

• Sensors type, range and what it is object/thing it is detecting.

• Fire control system type or yes/no and maybe some ECM burn through feats.

• Accuracy (usually in the form of they hit a target at X distance)

• Muzzle velocity of the weapon

• Any other odd quirks of the weapon that might relate to range (Universe sized projectiles? Only fires backwards through time?).

The Target

The target also has three major things that alter the attacker's effective range, ECM/stealth, size and maneuverability.

• ECM

Non-super science ECM is generally about making it so while the attacker might know in general where the target is, they can't resolve its exact location. Going back to my example with boxers, if someone is shining a bunch of 100 kW lights at them, they would have a hard to knowing exactly where their opponent is but it is obvious that they are where all the 100 kW lights are. Generally as range decreases the attacker will be able to burn through the ECM better and get a good targeting solution. For this you are going to want, what is it spoofing (e.g. a decoy missile that to mimic the ship's radar cross section IRL example: Nulka), how is it spoofing it (e.g. to the incoming missile the decoy looks like the ship so they try to engage the non-existent ship) and at what range has it been shown to work.

Stealth systems come in passive and active varieties passive (like the stealthy geometry of modern stealth jets) and active (Star Trek cloaking systems where they need to turn it on to get the system's benefits). There isn't a whole lot of difference in terms to what feats you should be looking for between the two different types but it is good to make the distinction as often active stealth systems can't actually be used in combat for whatever reason. Similar to ECM usually getting closer to the target increases the probability of the attacker burning though the target's stealth

You want to know how it is reducing the ship's "viability", what kinds of sensors it has been shown to work on, range it has been shown to work and whatever weaknesses the stealth system might have.

• Size

Size combines with distance between attacker and target to get the actual accuracy. This should be in terms of facing surface area but that isn't always easy to find so whatever size values you can get is OK. What this gets you is a minimum CEP (circular error probable) where at the range the shot was fired at it will likely end up in a circle which is at a minimum the diameter of the target's smallest facing axis. Though we should note with the exception of attacks against stationary targets with no ECM, size plus range isn't entirely enough information to get precise accuracy numbers out of the weapon as there are many other issues that can cause a miss vs a hit at a given range. But even in the case of a maneuvering target it still gives a baseline. Things like hitting specific subsystems at a given range would also point towards the CEP being quite a bit smaller than the smallest facing axis of the target.

• Maneuverability

In an ideal world this includes acceleration (ideally in all axis but that can be hard to come up with), roll/pitch/yaw rates, possibly top speed if it is relevant (it often isn't very relevant) and any other more exotic things (an example would be Culture ships dodging along a four spatial dimensions rather than the three available to most ships).

This is most relevant to the question of, how well can a target dodge in incoming shot? Or what is the dimensions of the target's probability cone (the 2/3-dimensional shape that describes every point where the target could end up within the flight time of the projectile)?

• Summery

What you want from this section,

• ECM what it is doing, how it does it, and range it has been shown to work.

• Stealth how it works, what it works on, ranges it has worked at and any weaknesses/oddities.

• Size

• Maneuverability (acceleration/roll/pitch/yaw and any exotic capabilities)

That is basically everything you would need to know to actually come up with some basic mathematical models to determine if something can dodge something fired at it from a given range and if it can do some basic probability to determine what is the probability of it being able to dodge random shots fired at where it could possibility be.