[Corsair]

I repeat myself, sorry, but the main concept to grasp is that you do no need to know your exact position on the map to plan an attack on open sea.
During that phase your sub is the center of the world. You just need to choose and mark any starting place when you get the first contact (which can be anywhere on the map or even on a sheet of paper on your desk) and from then on plot your moves and the position of other ships relative to your sub.