Paul,
 
I believe what Jim was saying was that, because most 3D reconstruction computers
assume a slice is as thick in the z direction as a pixel in the x-y direction,
you should multiply it by 6 times to make the z direction thickness closer to
what he has measured as real.
 
I hope he didn't say that his z resolution was 180nm, I believe that he said his
Z increment (step size) was 180 nm.  Our measured z resolution with a point
source fluorescent specimen on our MRC-600 is about 570 nm.