Maybe we should consider putting "difference between the best 2 measurements back on the cert form-- then it would be easier to collect facts about this.
I agree that it sounds needlessly restrictive in this way: it may be hard for a "newbie" to achieve that at first, and I wouldn't want to make the job of laying down a cal course too discouraging. But I can accept that it was decided, now I'd be interested in seeing data-- what differences do we ordinarily achieve in practice?