The optimal position for the origin is at the bottom of the hole in the link. In the image the order of the chain links runs from bottom to top, the origin is shown for the red link.

The links are spaced in increments of exactly 0.1 Poser Units. The thickness of the link (as viewed from the side) is 0.022565 PU, within a tolerance of +- 0.000005 PU. The width of the hole in the link is approximately 0.044 PU, the hight of the hole approx 0.1 PU. The overall hight of the link is approx 0.14513 PU, +- 0.000005 PU. By the way, finding this out was not made easier by the fact the P6 joint editor miss reports the position of the origin by more than  0.001 PU.

Attached above is the skeleton cr2. It makes more sense if you can see the actual figure in Poser, so I'll try to attach it to my next post.