1. I thought the gold ring around the sigma 8mm denoted the NP of the lens, if that is the case the 3.925 cmm seems to put the lens almost 1 cmm too far back - is this correct?
Probably correct. When shooting only two shots around, the images join at 90 degrees to the lens axis, so the entrance pupil must be visible when looking from the side of the lens in a direction at right angles to the lens axis. The entrance pupil is the point about which you need to rotate the lens, which will be well forward of the gold ring. See the photo at the end of this tutorial: http://www.johnhpanos.com/epcalib.html.