How did real Uboot sailors determine target range?

[Paul Riley]

I dont like the stadimeter much,much prefer eye estimations,as Hitman said,by judging how much of the target fills the scope you have a fairly good range estimation.You can also draw level with the ship,and track it on a parallel course,match speed,quickly plot your UBoat's position and your target,get the ruler out and get your range that way.
But as has been said already,range is not that important from a perpendicular position,only when using a gyroangle,like when firing from parallel positions,which would be a full 90deg turn by the torpedo (I am actually very successful with these attacks)