where U-boat firing solutions are concerned, range is only an issue in determining the time of travel so you have a "countdown" to impact against the target.
If i remember correctly, German U-boats did not make use of a stadimeter in the way that American fleet boats did. so their inclusion in SH3 is somewhat unrealistic.
In reality, German U-boats owed a lot of their success to the way they computed solutions to target by snap observations and quick judgements.
Generally, a target's speed and angle on bow would be estimated visually, as would the range. The only known variables to the solution which were 100% accurate would have been U-boat speed, course, bearing to target and speed of the torpedo and only because you would know these things about your own boat.
all things considered, with a German TDC, range had the least impact on solution accuracy compared to all other variables.
I highly recommend something actual German U-boat commanders used called the T.L.A.R. method. and it is something i use on my 100% realism campaigns in Silent Hunter III
The T.L.A.R. method is quick and simple, requires absolutely zero number crunching or lengthy calculations, it yields a reasonable result with an acceptable margin of error, and becomes more accurate the more often you use it.
to apply the T.L.A.R. method, first, observe the target through the periscope or UZO
then say "That Looks About Right" - then enter the value you felt looked about right into the TDC.
once you have completed entry of the remaining variables - Speed and AOB - fire the torpedo.
If you scored a hit, you have properly used the T.L.A.R. method
If you missed the target, continue practicing the T.L.A.R. method, you'll get it sooner or later
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