Manual torpedo attack

[modelrrfan51]

Hellguard,
How do you determine when to fire torpedos. This is one I just did and missed. If you have a ship coming from right to left and you are on a 90 degree course to target.
Ok, this is a aft torpedo shot. I put in speed first, 12 knots, then put in AOB, 90 degrees, then do a distance, 3600 yds at 250 degrees on periscope. Check attack map, showing torpedo track about 30 degrees from the 90 degree from stern of sub. So question is when looking through periscope, when do you fire, at what degree in periscope?
Recap, so target coming from right to left, you are 90 degrees from ships track. AOB is 90 degrees. Attack map show torpedo track about 30 degrees to right of the 90 degree of the track of the sub. So when do you fire torpedos looking through scope.