Okay so, what I have in mind for the new Stinger lock-on system: When fired, the missile targets the next hostile vehicle it can find in a 45° cone (Or maybe less, like 25°).

The only problem is I have no idea how to calculate if a vehicle is inside that cone. And since I suck at maths I need some help.
Why I don't just search for the next target? I don't want super-smart homing missiles that do everything for the player, and this way the player can choose a target.