..Wow.... Hard stuff..
I just do that 90 degrees off the bow, get the length of the ship, calculates the time it takes for the ship to run its full length, dividing length with time, and multiplies with 1,946. Voila, speed.
I must admit, back in school when we probably learned this hard math you are referring to, i was either asleep dreaming of some juicy convoys or i was drunk...