Well I brought into zbrush a 1x1 square surface from Rhino. It looks as if you have the Grid size at 1 then it is 1x1mm in the real world. If you go up to a grid size of 2 you have a square 2mmx2mm etc.
I noticed bringing in the 1x1mm square that it was centered in the grid. I exported it with a corner of the square in the center of the grid not the center of the square.
I also noticed that if I (for example) open up the demo head and then append the 1x1mm square then the 1x1mm square is where it should be with a corner at the center of the grid.
Is there a way to make this consistent with how I export/import it?