I haven't tried it long enough with side angles and difficult aiming situations, but does it actually mark shooting directions when ships are turning and you have a side angle where you need to aim at the air in order to hit the enemy? Also, does it show aiming upwards/downwards angles too, like for a mortar? Because if yes, then it can be quite useful. In all cases I've tried it so far, all it did was putting a circle around the ship I was pointing at, which wasn't really helpful since we all saw the ship itself, and some gunners found it more distracting rather than useful to have a glowing circle around enemies.
Also, it would be cool if the tool could work not only when somebody was looking through it, but let's say, when somebody selected it, because as others above pointed it out, the idle engineer/captain is a rare case, no engi has time to assist a gunner's every shot while the ship is falling apart. At this point, the tool is not really useful for gunners, since you cannot shoot and use the range finder at the same time. (The wild idea occured to me to make this a tool that would work on gunners automatically if a gunner mounts a gun and brings this instead of the spyglass, and everybody could see range and all later on, or maybe just the gunner, but he could communicate it. Could perhaps make gunner class more useful in game.)