Further to my last posting, these are the page setup parameters with which I cannot get exact mirror registration. I'd be interested in learning if anyone else can get them to work:

MARGINS
Top: 26 mm
Bottom: 30 mm
Inside: 57 mm
Outside: 17 mm
Gutter: 0 mm
Gutter position: Left (disabled)
Orientation: Portrait
Pages: Mirror margins
Apply to: Whole document

PAPER:
Size: Custom size
Width: 173 mm
Height: 246 mm

LAYOUT:
Different odd and even - SELECTED
Different first page - SELECTED
Header: 12.5 mm
Footer: 12.5 mm
Vertical alignment: Top