I use Trigger Maru so I don't get spread lines, x's, ship silhouettes or any of that. I get information much like that a real submarine would plot.
If I remember right, the x is the target point of the TDC/PK. You want that x to end up on top of your target. If is is on the torpedo track line crossing the target but closer or further away, your range is wrong. If your PK is activated and you observe the line for as long as the torpedo will take to get to the target and the line still intersects the target, you will hit anyway, in spite of the error.
If it drifts off the target in that amount of time you'll waste a torpedo. Then you can do one of two things: correct the range or alter the speed so the track line stays on the target for as long as you need to hit it. You might need some trial and terror to accomplish this.
With Trigger Maru you'd better just make sure all your parameters are right. It demands more precision than the stock game, by far. That is because it has arcade features.:rotfl
inside joke from another thread)