When I was working for David Pedrick designing 12-Meters for the America's Cup, we were looking for hundredths of a knot.
One one hundredth of a knot gets you about 60 feet in an hour . . one boat length for a 12-Meter. That's all it takes to win a race. Gain three or four hundredths and you have a nice cushion over the competition -- assuming you don't flub a jibe set, of course, or sail to the wrong side of the track.
For us cruising folk, a plank will probably serve just fine