I close to about 2000-4000 before i fire. When i do fire, I set it up so that i either
1. Take out as many targets as possible
2. Make sure that i take out my one target
3. Can disable the most dangerous target
I have disabled several destroyers and then followed the High Value Target ships a good while and sink them. But, the third option is a bit harder than the first two.
A torpedo to the screws can do a lot to a destroyer.