With increasing precision, perfection from the measurer, reading a jones counter to 1/4 of a turn, a well practiced rider who wobbles least, fair weather with no cross winds and a perfect alignment of the planets, I think you can use much shorter courses.
The point of having a universal way for mesuring is that it can be executed by an inexperenced mesurer, under less than ideal conditions, using the device to the resolution of the device and not beoyond.
We are aware of the un-preventable start and finish wobbles, the whip lash when backing up a jones counter and the number of other errors that an inexperienced measurer may miss.
By having a calibration course that is longer than mathematically necessary when conditions are perfect helps eliminate a lot of the end errors and get more of the middle counts that are hopefully similar to what the rider will do on the real course.
I think there is also a difference in riding when balancing down a 300m course, trying to get started right and then immediately switching to finishing right, rather than doing some heads up riding as one expects to do on the real course.
It takes me riding my 1/2 mile cal course a couple of times to warm up my bike, my tiers and my muscles before the counts start to be constantly repeatable. Maybe this is just my inexpedience. If I was working on a 300 meter course I don't think I would never get to the same stable state as when I am riding a course.
Now it helps that I am not traveling to do measurements and have the advantage of a 1/2 mile totally straight and flat road. Obviously the cert course does not have to be 1/2 mile long but at that length I feel very confident that any minor errors, like counter whip lash and bobbles at the start and finish are only multiplied by 6 times when scaling up to a 5K course.
There is a direct mathematical relationship between the length of the cert course and the final race course. What ever that ratio is, any errors that get introduced in riding the cert course will be multiplied by that factor. The shorter the cal course the more times error will be multiplied. Yes the percentage of error stays the same, but it's that start, stop, whiplash and rounding error that is minimized with a longer cal course. Taken to the logical limit, the best cal course is the same length as the target distance.
Even though I run on very hard high pressure tires, 100 psi, and warm up myself and the bike before starting on the cal course or the race course, there is still a difference between pre and post calibrations. Even very minor changes in the weather can make a measurable difference. I can detect the variations becuase my cal course is long enough for the variations to have an effect. With a much shorter course the changes would be masked by rounding errors.
Remember, we are trying to perfect a method that while not fool prof, is going to allow an inexperienced person who follows the method to arrive at a course that will pass verification.
We are under attack from people who want to replace our mesurement system with GPS or other ideas. It is not good when the 'Method' we use, when in the hands of someone who is not a high preast, leads to compounding errors and brings the methodoligy into question. Same reasion we don't allow a mesurement done with a yard stick.
I understand the math and sound logic for your argument for a shorter course, but you are working from data derived by a very experienced and careful measurer who is taking extraordinary care to measure and record to a higher precision than the jones counter reads.
The methodology must be robust, and must be robust in the hands of an inexperienced operator who is following the instructions for the first time. For this I think you don't want the shortest cal course possible.
Maybe we need some stats of first time measures, who are doing it on their own for the first time, without the supervision of an old hand who teaches them tricks and points out their errors.