I determine it the same way I do my own - fuzzily! Hitting easy/small things is often good, it can remove or add the Fleet Size Bonus as well as what Oliver mentioned. But if I have a bunch of shots (say 6 CAs) I might shoot at CA before SC/DD, then when I have fewer I will only shoot at the CA if one is still damaged. Weigh the difficulty to hit versus the offence the ship in question has.
Yep, this is pretty much the mantra I follow as well.