[jason210]

I look at the ship through the periscope, and time it as it passes a reference point. This fed into a slide rule gives me the speed of the target.

I estimate the relative bearing of the target, then use a physical whizz wheel to feed in target speed, target bearing and torpedo speed to get the right torpedo fire angle.

TDC is off, and manual entry on (green button). This is so the gyro angle does not alter when I move the periscope.

The hardest part is estimation the relative bearing of the target, because it can be very deceptive through a periscope. I haven't figured out a way to reliably and accurately measure that yet. However, since the ship mast height and ship length can be expressed as a ratio, then it should be possible to derive the angle of the ship by comparing the apparent ratio with the actual.