JamesM has a good idea hidden in his post, which I think could be added to the measurer's manual, which first-time measurers should be reading. That point should be that when conducting your calibration rides, you should ride the cal course at least twice in each direction, and until your counts in each direction are within 2 clicks (ideally one click) of the other ride in the same direction.
I believe this will encourage newbies to pay attention to wobble, and will let them know what an acceptable multiple-ride variable is. Currently, the only instruction is to ride twice in each direction, then average the counts. It says nothing of accuracy and repeatability.
I do, however, disagree with JamesM that 300 m is too short to avoid wobble. I think 300 m is a viable length. I compare my cal counts from multiple rides, and they are very consistent.