Tank v tank you need to guestimate distance and shell drop and sometimes when a long distance need to get a test shot in to gauge range. Sometimes you know where someone is (as they just killed you), you grab a tank and let rip with a shell (yes I know I was firing from within base and I did feel guilty but I wasn't red then so it was OK and it was only laka) and you get the lucky kill and swear a lot because you were not recording! Movign jets and helis are harder as you have to do a quick mental calculation to allow for not only shell drop but also estimate where they are going to be too.