Hi Daniel,
I'm thinking there are a number of work-arounds, none perfect, but maybe get you out of a hole
1) do not use the printer's paper cutter. (It's another consumable to be replaced.) Cut manually. If you are stitching the prints together, overlap and cut the matching edges at the same time.
2) in the printer manual, it will explain how to adjust the cutter position. For you to manually measure the length of flexible paper or canvas to less than 0.25% accuracy is not necessarily simple. For a mechanical device relying on feeding various media through rubber rollers, sometimes dragging it off a heavy roll, it is something else again.
3) print at 448mm length, but cut at 449
4) try printing in some other software. If that is cut OK then possibly use that, it will show there is a problem with qimage, or something odd in your qi settings. If that other software has the same trimming problems, then it looks more likely that your hardware adjustments are required (or you've messed up this software settings too
).
5) read up on borderless printing and overspray.
Without knowing exactly what you are trying to do e.g. stitching images for a panorama/whatever, and whether it is one offs or dozens or more, it is difficult to give a suggestion to get you to achieve your final objective. Personally, I would never expect the effort of precisely setting the mechanical cutter to be worthwhile - but then I'm not doing masses of batch printing.
Best wishes,
Ray