Where to shoot the Destroyer with a Battleship ?

If im in a Tier 6 BB lets say the Italian T6 F.C. and i shot a T5 DD broadside at close range (4-5 km) with HE loaded and only take of like 20-30% health, i must doing something really wrong because when im in a DD and have that situation, I usually get 75% health off or direct deleted 😁

To make the most damage where do you aim?