When it comes to max no-escape range/min safe distance I prefer a little dirtier of a method.
Torpedo gain rate * torpedo run time. Let's take my favorite example first...
Torpedo gain rate: 55 knot Mk48 adcap gaining on a 43 knot Alfa is 12 knot gain rate.
Torpedo run time: 55 knot Mk 48 can go 23(?) nautical miles = 25 minute run time. ((23nm/55knots)=.4 hours*60minutes=25minutes)) (For figuring safe distance also consider the # of minutes it will take you to put the torpedo directly behind you and reach top speed.)
Gaining at 12 knots for 25 minutes = 405 yards/minute * 25 minutes = 10125 yards max no-escape range on an Alfa.
Now assuming for some reason the target is a Sturgeon or Ohio or Rubis(?) with 25 knot top speed.
Torpedo gain rate: 55 knot Mk48 adcap gaining on a 25 knot sub is 30 knot gain rate.
Gaining at 30 knots for 25 minutes = 1012(!) yards/minute * 25 minutes = 25300 yard max no-escape range on a 25 knot boat.
Ok but what about the poor Alfa skipper?
40 knot TEST-71 can go 15km/8.1 nautical miles for a run time of 12.2 minutes.
40 knot TEST-71 - 33 knot 688 = 7 knot gain rate
7 knot gain rate * 12.2 minutes = 236 yards/minute * 12 minutes = 2879 yard no-escape zone. Poor Alfa skipper
So the finally tally.
Mk48 no-escape range against Alfa = 10125yards
Mk48 no-escape range against 25 knot boat = 25300 yards
TEST-71 no escape range against 33 knot boat = 2879 yards
Did I overlook anything? Let me know