I'd really like the option to disable the annoying 'aim-assist' circle when manning a gun. It's hard enough trying to pop someone's balloon from 2km out without people projecting glowing dots all over the place and possibly blocking my view of the target.
For people who know how to play the game, I would say the rangefinder is (somewhat counter-intuitively) most useful at ranges around 1km or less, with a special case for things like lochnagar lj/hf and carronades. Mostly, the guns that have projectile drop also have very long shell life, so it's less useful for those- it works best with guns that have no projectile drop (rockets, carronade pellets, gatling bullets). The most frequent case is finding exactly when the enemy gets within the 1200m mark, so you can instantly open fire with the artemis and be sure to hit. You can look at the other guns' ranges on the website, and if they're wrong bug muse to update them.
The true sniper's mainstays- lj, h. flak, merc, possibly hades- all have very long shell lives along with substantial projectile drop, giving them virtually unlimited range. Rangefinder is unnecessary, but they do require substantial skill/knowledge to predict how the projectile will act once drop becomes a significant factor.
p.s. muse, if you want the rangefinder to work with the true sniper weapons, change the aim-assist UI from a circle into a single dot (like the one used in space sims such as Descent:FreeSpace), that very clearly says, "If you shoot in this direction/angle and both you and the enemy maintain current vectors, your shot will hit", instead of the current "You are pointed in the enemy's general direction. Here, have a circle around them in case you didn't notice". However, such a system will probably be difficult to implement because it will have to account for muzzle velocity, wind, projectile drop, ammo type, and the speeds of the origin and destination ships. A tool like that will also probably render all my hard-earned LJ sniping skill moot, but that's a different issue.