Yes, those lengths come from there
Well, i don't know, but i've used this method for a long time now and i've never missed my target, always hit the ship in the center, never miss, unless the torpedo is dud.
I'd say this method is accurate enough to take a ship to the bottom and i'm very pleased with it.
And this method is much more reliable than "Auto speed" in TDC or guessing.
Good hunting!
Happy New Year sailors!