i think the spread button is unnecessarily complicated to use, since as magic 452 says, the angle settings become very dependent on range. The formula is quite complicated to get right (although it can be done) and adds an extra layer of difficulty when making the firing set up.
fyi, the formula is here
where coverage of 100% means the spread will exactly match the target length, 80% will mean if accurate each torpedo will strike the hull somewhere, 150% means the spread is wider than the target length (useful if being sure you want to hit the target), tta is torpedo track angle, ie the angle at which the torpedo will hit the target.
much easier is to use a longitudinal spread which magic452 generously attributes to armistead.
it looks like this
it's easiest, and the best to start with, and its effective, but if you are spotted and the target turns away enough to miss the first torp, then it will miss all the torps. (PK should be off for this method)
other forms which cause more divergence and therefore are harder to avoid (but more work to set up) - in these there is no need to use the spread button, just aim the crosshairs at the right part of the target and send bearing data to the tdc just before firing
all images from the submarine torpedo fire control manual 1950
http://www.hnsa.org/doc/attack/index.htm