With practice it's almost second nature with a guesstimate.
But for numbers... torp runs 44kn or 81.4km/h (1knot = 1.85km/h)
So in 1 min it travels ~1.36km. or 44 seconds for 1km.
Soooo, if your ships are 1 km range difference, wait ~40 seconds before firing off second torp.
I find that's the easy part... the hard part is not having another ship cut across the torp path and triggering it early and saving your target
T.