Just be aware of the TDC torpedo speed glitch. Always click on the speed knob when dealing with torpedoes, even if there is only one speed to choose from. For some reason the game will use the wrong speed for the lead calculation, if you don't always do that.
http://www.paulwasserman.net/SHIII/
Fast 90 instructions are probably the easiest way to understand how manual targeting, works....
With the fast 90 technique you're predicting the 90 degree AOD.... and working backwards from there, which makes the overall process a lot easier.
And GWX added compass makes calculating bearings much easier than using a calculator... I don't know if you're using GWX, though.
Fixed wire is a way to calculate the speed of a given ship by knowing the ship's length and plugging it into a formula.... that involves time. (will need a calculator, for this...). I don't remember the formula.
Anyway, it eliminates the need of having an accurate ranging distance. Which happens to be the hardest thing to do in the sim, due to the recognition charts of mast heights being so hard to depict, the errors of constantly bobbing, the awkward aiming controls, and other such things.