Jake's mention of the dangers of being last are true. In the last Outer Banks 500 (2004), we were neck 'n' neck with another boat when they jibed away and went over. We kept going for a while knowing they were experienced sailors. After about 20 mintues, when their sail didn't pop back up, we turned around and went back for them.
The crew had missed the trap wire with his harness and went overboard. With the spinnaker up and windy conditions, the skipper couldn't keep the boat down and eventually capsized but not before putting a big gap between himself and his crew. They were more than a mile off shore and being seperated farther by wind and waves. We pickup up the crew and deposited him back with his brother and their boat.
In that case, I got a range mark on land and hit the timer on my watch when we first turned around. I think the whole thing took about an hour. We asked for redress in the amount of time that elapsed to rescue the other sailors and return to our range mark. The next day, we went over and it took about 15 minutes to get the spinnaker untangled and the boat righted. Another boat that was about 10 minutes behind us altered course slightly to check on us, which was sportsmanlike of them. But they also asked for an exorbitant amount of redress, which was not sportsmanlike.
So what's fair? Grant redress in every case to encourage competitors to watch out for eachother, or demand some kind of proof that you were, in fact, hindered in your quest for the finish line.