Obviously you want to be as close as possible to the helicopter when you fire. The RPG projectile is modeled to randomly change directions after 200m so even if you have the ability to track a target accurately you are limited by the how the game is coded. Remember helicopters are fast and your projectile is only slightly faster...so depending on the angle, you might have to lead a lot more than what you intuitively feel.
@maxdakka7973