I have been meaning to set up a downwind mark (which you can "project" from your position directly downwind if you can figure out how to do it on a foretrex) and test that issue by doing both and only looking at your VMG. I believe it also depends on wind speed too and also if there are significant shifts. Guess it would have to be a pretty significant shift early on the downwind run to make a difference but it would come into play especially if you overstood C mark and need to run high to get there. But then again... I never can pick out wind shifts. I have been crewing a lot recent with a good skipper and have tried to read the wind a little better but it still seems like higher math (or biochem) and we never really got along.