One inch in 1000 feet is pretty good agreement, about 1/10000. Two inches (1/5000) is about the edge of decent measurement with a tape.
As for the half mile calibration being more accurate than 300 m, that's true, but the difference is insignificant. If you had 10 people lay out a race course using a 300 m calibration course, and another 10 use a half-mile cal course, you would be hard pressed to find a difference between the two resulting courses. There would likely be more difference due to differing road surface roughness than length of the cal course.
The 300 m minimum length was brought in to encourage people to lay out calibration courses close to the race course. Too many people were driving 60 miles to a half mile cal course because it was too time-consuming to lay one out, even if you could find a straight half mile near the race course.