I should add that it does this even when I use a 20x20 on a 24" wide paper so this issue is not related to having an image that needs to be scaled.
See my last post in the other thread about trying it without the poster size, but if the above is true (even when you don't have a poster size), then
there is a bug in the printer driver where it is not responding properly to paper size changes outside the driver. Because I can't change the driver, the solution to that would be to not use a page size that is too big so that you have to trim it: use the correct paper size up front.
That said, this still sounds like a problem with scaling in the driver. Per the other thread, you have to select "print image with actual size" when you select borderless otherwise the driver is going to be doing scaling. What happens if you don't select that option is that as soon as you give Qimage Ultimate the command to remove white space, QU sends a command to the driver telling it to reduce the paper size: then the driver sees the new paper size and it RESCALES the image to try to fit the new size. And therein lies the problem.
Mike