Learn to use the depth setting on each torpedo with the known depth of the keel on the target. I run about 1m below keel, and always aim for the engine room. Bow hits are supposed to be effective, but I haven't noticed any real difference, the torpedo that hits the stern floods it as well, but probably at a slower pace.
I have found my torpedo's cover more targets, using ONE per target. It make take a bit of stalking after the hit to ensure it's going to go down, but I find alot of 'bigger' ships lose propulsion, and eventually sink. However, unless I'm freaked out by the tonnage of a perspective target, I'll fire all four forwards at one, hoping 2 will hit and cripple it allowing for a stern shot.
Had a 4 stacker ocean liner troop transport come up fast, and I was about 2000m away from where I wanted to be for my attack - all I could do was fire all 4, and yep - 2 hit, slowed it way down, and the stern fish finished it off
. Wasn't going to let that guy get away.
About the green triangle - you're talking about where it locks onto the ship? I rarely go by that, unless it's a really fast decision to fire.