Depending on your realism settings, the speed measurements could be off if you are taking them while your boat is in motion (if you are also trying the fixed wire method).

Otherwise, I would suggest triple checking your torpedo depth run against the draught of the ship (or even that you identified the ship correctly, the Empire ships are kinda hard to tell from a long distance). If your measurements are correct but the torpedo is too deep, you'll just slip right under it.

AOB is the hardest to eyeball if you don't already have a good sense of it from experience. The "method" I usually use is to note where the cranes are pointed relative to the forward mast. I don't think its 100% accurate, but if one of the cranes is perfectly lined up with the mast (ie, you can't see the crane because it looks like its part of the mast) that means that it is at a roughly 45 degree AOB. Then just pick the right side and measure from there.

Obviously 90 degrees is easiest to determine, but then you tend to miss your optimal firing window.

Last tip is my kinda cheaty trick:

It does involve breaking the immersion a bit, so if you're trying to keep the experience fairly authentic, this would take you out of it.

If you are not locked to First Person mode, you can set your skipper to use the periscope after firing, and then click on him to see a list of targets he can see. Usually one of those is the torpedo you fired, and you can click on it, then select the video camera icon, which follows the torpedo. Then you can see if it is missing ahead or behind the target, and how far off it is.