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Which version of Poser are you using? If you have a recent version of Poser, one that supports 'Animatible Origin', then I suggest the following.

Forget the Joint Editor, don't use it for positioning the origin, or only use it to get an initial rough approximation. With the item selected, go to its Properties tab, and enable 'Display origin' and 'Animatible Origin'. Change back to the Parameters tab and use the Origin dials to position the origin. If you still do not have fine enough control, you have two options.

1). You can click on the numeric field of the Origin dial. You can type in a value to a precision of eight decimal places (In Poser Native Units. It may be different in other units).

Or...

2). If you double click on an Origin dial, you will be presented with a box where you can change the 'Sensitivity' of the dial. The Sensitivity (aka 'trackingScale') sets the smallest increment of change when you turn the dial.