I'm not sure what a good compromise would be. I don't usually play S-boats, because I've been using the 3D Radar/TDC mod.
Some ideas:
Use the TDC, but leave the Position Keeper section turned off. Enter range, bearing, speed, and let the TDC calculate the lead angle, and then fire when you and the target are at the correct positions.
Learn the math, and how to figure it out. Program it into a spreadsheet, or break the problem down to some simple quick calculations.
Use pencil and paper to draw out the problem, and solve it on paper.
http://www.valoratsea.com/notdc.htm
Now you've got me thinking of starting an S-boat career. Could be fun.