[Nisgeis]

Right, it's a bit of guesswork and a bit of science. Which information do you have and what information do you want?

OK, so assuming you know how to use the omnimeter to get the target range from the mast head height and telemeter scale, then what you need to do is to reverse the process to get the foreshortened length in feet of the target based on the angular length. So if the target was at 2,000 yards and it spanned five horizontal graduations on the telemeter, then set 5 against 2,000 and the indicated mast head height is the foreshortened (due to the AoB) target length.

Now, if you know what the target length is, then you can set the 90 (with the black arrow) against the known target length (using the range in yards scale) and then where the measured target length in feet is (read as against range in yards) then that's the AoB.

Or, if you guesstimate the AoB, or plot it to work out the AoB, you can deduce the real target length by placing the plotted AoB against the foreshortened target length and where the black arrow at 90 is will be the true target length.

Target length can then be used with the Torpedo Spread Angle Calculator to work out what spread angles to use.