Hi Lee,
Not exactly sure but I think its around 44.5mm.
Just work on the distance from the front of the T thread adaptor (side with the screw thread) and add 55mm from that to the ccd sensor. As all T thread adaptors are made so that the focal plane is 55mm back from the front surface. So if your Nikon focuses there should be no probs with other major brand cameras..
Cheers Gary