Lyman,
The "slop" I was referring to is the face that if you roll backward to a stop, when you then start rolling forward the jones won't start moving until you have rolled forward a good bit. This is mostly due to the loose connection between the tang of the JC and the spokes, but is also due in some small way to the slack in the gearing of the JC itself, as you say.
But it sounds like what you may be talking about is the lack of precision in reading the Jones count on the dial.
But the way you can eliminate both issues when you calibrate is to mark your wheel and calibrate based on wheel revolutions rather than JC counts.