The most fair method is to anchor to the left side (looking upwind), setting a line that is no shorter than 6 lengths (mark room on both sides), and is square.
I don't think there is any way to really make a shortened course fair for everyone.
By then the race is already a crap shoot and you are just trying to save a race that otherwise doesn't prove much.
The only fair way is to toss it, but those who are leading will feel cheated, of course.
Rarely when there is a shortened course do many or any of the racers actually know what to do, making it even more of a crap shoot.
Just my opinion.
