Digital BW, The Print

I  am making some sort of error trying to get a specific print size out 
of qimage and qtr.
I want the print to be 744x480mm printed on canvas. I print to file 
from qimage with a page size of 746x482 so that there is a 1mm margin. 
When I have printed to file I check the print in photoshop and these 
sizes are correct, so I can assume that the problem is not a setting in 
qimage.
I then load the file into qtr with a page setting of 800x610mm to print 
on an Epson 7800. The print is centred but not fit to page as there is 
plenty of room.

The print size comes out at 728x478mm - much more reduction in length 
than width. Could it be margins? there are no warnings in qtr. It could 
not be the canvas stretching simply because I can not streched it back!

Has anyone any idea what I am doing wrong?

Regards
John White