If my torpedo did massive white damage then I shoot the decks and bridge with my deck gun to finish off the white bar.
If my torpedo did massive blue damage then I shoot the water line with my deck gun to finish off the blue bar.
Whichever bar is closer to zero is how I decide how to aim my deck gun.
If it is a war ship I tend to aim for the guns and bridge as disabling them are my priority. Recently I was flanking away from a convoy with a destroyer giving chase yet he couldn't do anything but chase because his two front guns were ablaze thanks to my gun
Once the fires hit the gun magazines he was toast.