OK, I'm back. Now I'll give you the long answer if you're interested as to where I got these numbers.
First, keep in mind that the Type I torpedo has three speeds: SLOW (30 kts), MEDIUM (40 kts), and FAST (44 kts). Most other torpedoes run only at the SLOW speed of 30 kts. I always carry a decent number of Type I torpedoes because I like the fast speed (smaller lead) of these torpedoes.
Second, I should note that I only use the "snap shot" in close -- 600 to 1000 meters. If the target is at range, I figure I always have time to do a decent TDC solution.
So here's an example and you'll want a Type I torpedo selected to do this. No target is required for the demo.
Make sure your periscope is pointing straight ahead -- 0 degrees relative. Next, go to the Attack Map (F6) and turn TDC OFF; set Target Range to 2000 meters and Target Speed to 0. Now the torpedo run line on the Attack Map should be straight out the bow of your boat if your periscope is pointing straight ahead.
Set the Type I torpedo speed to (S)LOW.
Now set the Target AOB angle to 90 degrees (right or left makes no difference) and set the Target Speed to 6 kts. Note that the torpedo run line is now 13 degrees left or right of your nose (depending on whether you set left or right AOB). This is your lead angle.
Now run the range up and down. Note that the lead angle does NOT change. As stated by misha1967, this just illustrates that Target AOB and Target Speed are the two most critial elements in your lead solution -- Target Range is not.
Put the Target Range back to 2000 meters and change the Type I torpedo speed to (M)EDIUM. Now notice that the torpedo run line (lead angle) is about 9 degrees. Change the torpedo speed to (F)AST and note that the run line (lead angle) is about 8 degrees.
So what have we learned ...
For a 90-AOB shot against a target going 6 kts the lead angle is:
13 degrees for SLOW torpedoes (approximately 2 x Target Speed)
9 degrees for MEDIUM torpedoes (approximately 1.5 x Target Speed)
8 degrees for FAST torpedoes (approximately 1.3 x Target Speed)
... and Target Range has negligible effect on the solution unless target bearing off the bow (stern) is more than 30-35 degrees relative -- remember, the torpedo straight run issue.
You can repeat this for different Target Speeds ...
For Target Speed = 9 kts, the lead angles are 18/13/12 degrees for Slow/Medium/Fast torpedoes respectively; and for a Target Speed = 13 kts, the lead angles are 26/19/18 degrees for Slow/Medium/Fast torpedoes.
... and you'll find the relationships hold pretty true for a 90-AOB "snap shot":
SLOW torpedo lead angle = approximately 2 x Target Speed
MEDIUM torpedo lead angle = approximately 1.5 x Target Speed
FAST torpedo lead angle = approximately 1.3 x Target Speed
Now, if you really hot, you will remember that ...
For AOB = 60/120; sin(60) = .866 (I use .9)
For AOB = 45/135; sin(45) = .707 (I use .7)
For AOB = 30/150; sin(30) = .5
... and apply this multiplier to your solution if you don't have close to a 90-degree AOB solution. Just remember, because of the torpedo grazing angle, if you're going to shoot much off the 90-degree target crossing angle, you will want to go under the hull with the influence fuze. In general practice though, applying AOB adjustments requires too much brain power for me
-- which is why I usually reserve the "snap shot" for close in, close to 90-AOB situations ... but it does come in handy for a zigzagging target, in which case, I usually stay close to the target; pick a solution; and wait for the target to give me the AOB I'm looking for.
Just don't forget that Angle-Off-Bow is your relationship to the target -- your position off the TARGET's bow.
Good luck!