Most printers have a tendency to print slightly shorter than they should on certain media. Canvas in particular is troublesome, particularly as canvas prints frequently have specific length requirements due to the need to stretch them on stretcher bars.
Earlier I printed a 16x20 canvas, on a 20x24 piece (losing two inches on each side for stretching). This prints sideways on a 24-inch roll. Unfortunately, the canvas, while 24 inches wide, was only 19.75 inches tall. While this might be acceptable at this size, it will scale up and make a 40x60 print utterly worthless, as it will be unstretchable. Being 1.25% too short is something that must be fixed.
While one could fix this manually in Photoshop before printing, it would be excessively tedious. My Canon iPF8100 has a per-media-type length adjustment. The quick calculation is that to compensate for a 1.25% shortness, one must make the print 1.27% longer. In the length adjustment field for the media, percentages are in 0.02% intervals, so I adjusted it to +1.28%. After adjustment, the new 20x24 canvas now measures closer to perfect than I can measure.
Note: For any iPF users, the media length adjustment is only applied if "Feed Priority" is set to "Print Length". Otherwise, it has no effect. Additionally, length adjustments are iterative. That is, one can set the length to anywhere between plus 0.70% to minus 0.70%. Then, once one goes back in to the setting, it is still at 0%. If you were to set the length adjustment to +0.15% twice, the true adjustment would be +0.30%. In order to set it back to zero, you would then need to enter -0.30%. As far as I know, the only place to check your actual setting is in the Status Monitor application, under Information->Status Display.