1st. from 0 - 7 km a battleship should be able to delete that destroyer.
2nd. from 7.1 - 12 km a battleship should get maybe 1 pen,and the rest over pens.
3rd. from 12.1 or farther away maybe 1 over pen and splash the rest.
I am a BB main, but I play a little of everything. If a battleship is 0 - 10 km away he will get torped. If he is 0 -16 km away he will get either get torped,or set on fire. If a battleship sits any farther the the battleship player gets reported for playing poorly and not supporting his or her team.
At this rate why even play a battleship at all? Maybe just remove battleships all together. Wargaming prides themselves on trying to be accurate with their ships and specs. In the real world a destroyer 5 km away would get deleted by a battleship. Next destroyer players will be crying that cruiser HE does too much damage.
Just so you know I also play destroyers,and still feel this way. Wargaming needs to make it FAIR , and FUN AND ENGAGING.