I found the alt + rightclick move to be very useful. I had wondered how to do this before. That way I can have my ships focus fire on enemy ships without completely messing up my line, which is, as others have mentioned, already hard enough to maintain.

Interesting results happen to me, when I just start the battle, select all my ships and hit the line formation button a few times. They will align at 90 degrees to the axis, they will turn around completely and have a new head of the line. Good way to completely confuse your formation before the french hit you!