How do I act quickly with manual targeting?

[Musician]

I'm brand new to the silent hunter series, and realistic simulation games in general but thanks to the video tutorials here, I've really been able to take to manual targeting. The other day I was able to take a merchant out using only sonar thanks to the advanced targeting solution guide on this site.

My only problem is, when I'm not expecting an enemy, and I'm not prepared and at a good optimal firing position, and I don't have alot of time to make calculations especially when there's bad visibility, what am I supposed to do?

let me give you a situation, I'm heading towards a convoy and a warship notices me and starts heading directly towards me at about what I estimate (roughly) to be 25-30 kts. In this situation it's also raining and is very dark so I'm unable to identify what kind of japanese warship it is. Since it is heading directly towards me, should I choose to fire a torpedo I'd have to hit a very small target which is out of the question for a ship that I can't identify and is taking evasive manuevers... (this just happened to me :/)

so what am I asking is: realistically, how should I deal with a situation with this so that I ultimately get a kill?

thanks a bunch