I've attached the verbage below. The way I read it, if there are ships from both sides still on the board, then the winner is determined by the percentage number of "units" (in this case - ships) lost. Except for breaking a tie and picking a best player on each side, the league system doesn't seem to look at the actual points scored.
Anyway, I'm about to find out how this works in one of my games.
Ogre
If, at the end of the game, there is more than one side remaining on the board, the victory could be tactical. This happens when the number of enemies destroyed is percentually more relevant than the unit lost.
E.g. Suppose the leagues is based on a 4 vs. 3 scenario. If at the end of a game there are still 3 units of side A and 2 unit of side B, then the side A achieve a tactical victory since side B has suffered a numer of loss percentually higher than side A. On the other side, if the game ends with only 1 unit still on board for each side, than the winner is side B, since thay have lost onyl 66% of their units, while side A have lost 75% of units.