You misunderstood the problem.
I had told the printer to use a page size of 16" wide by 24" long. That would have been OK if 16" roll paper was loaded. But I had 24" roll paper and should have picked a page size in the driver setup of 24x16: 24" wide to use the 24" roll paper, and 16" of length.
The image was added to the queue with fit to page, so the 24 vs 23.766 wasn't the issue. I was using 24x16 in nominal terms in my description. But I expected it to be the smaller 23.766 x 15.766, and indeed that is what QImage gave me.
The Epson driver and Qimage did exactly what I asked for. But not what I intended.
The suggestion to always use the driver's preview would have caught it. My query was to see if now or in the future QImage would have some way to warn of this problem.
Tom
Sorry, Tom, but I doubt if I got it wrong. You wanted to make the 24" side of your print fit the width of the 24" roll/
The printable area when you set the page size to 24.0 is only 23.766.
You cannot fit 24 into 23.776.
So to utilize the entire 24" roll, the user, must tell the Driver that the page is 24.02". Then click Borderless.
Now the reported printable area is 24.01 and the 24" print will fit across the 24" roll.
You aren't the first one to come across this.
As for a warning, you have one! Check the printable area first.
Second warning: If you told Qimage to place a 16 x 24 on a smaller printable area, it tells you it needs more paper by displaying a red 2x or 3x on teh preview panel.
Good luck,
Fred