Thanks a ton for the input, you all!
This is an attempt to find out if it is at all possible to increase the probability, to find out how much margin for error I actually have, theoretically up against the game and what it allows.
Questions are, how much inaccuracy for a 100+ metres ship? How much error can be allowed to hit within 40 metres to either side of centre? And how accurate does the game allow me to be?
So far I have found the stationary 4 bearing method by far the best but it does have limitations on accuracy if I disable the hydrophone lines and even they are not telling the true story, sometimes pointing near the engine room, other times near the screws. Without the lines I have to allow for 1 degree error on each bearing and that could mean getting the target course wrong by as much as 10 degrees or even more and reading speed is no better off when I need that much higher accuracy. I am looking at the possibility of using optics at closer range to get more exact bearings but unsure if it can be done within the game, in particular the fact that I can only read a reasonably accurate bearing via the TDC and need to jump to nav map while time is ticking.
As for timing the fireing, at those ranges it's impossible to be exact, resolution and how much each pixel is in ship length means I have to eyeball it so there is inaccuracy there.
I already have found that surface attacks are right out. Only by staying submerged can I keep the course at a constant heading and even then it takes a bit of rudder to keep it at an exact heading. Submerged, it's manageable but I don't want it worse.
Steam torpedos and wake, that is indeed a challenge. What I wonder is how to decrease the probability of them spotting it. Lighting, weather and their crew experience matter but what can be done to maximize the chances there? Reason I wonder is I had ships taking evasive maneuvers at night when my steam torpedos were some 1500 metres away. Just bad luck and the A Team on the bridge? I really would like to know.
Outgoing targets, I'm sorry but what do you mean by that? Targets past the fireing angle?
I depend on target keeping a constant heading and speed. Without that, I don't think I could guaranty a hit at 400 metres

let alone at 10,000 metres.