*ImageableArea sets the area of your image to capture and print.
It does not set the printer page area.
Here is a sample image that gets cut off on the left side when printed in landscape on my Canon MG2900.
This happens because this printer needs a 2/3" margin at the bottom of page, or the left side of the rotated landscape page. In other words it can't print on the last 2/3" of the page.
In order to print this page fully, without cutting it off, it needs to be squeezed down a little so it will fit within the area that this printer can print in. (Unfortunately, scaling it down didn't fix this. It would move the right text in, but still cut off the left side of the page.)
To fix this, you set the ImageableArea to the part of the image that you wanted printed.
In this example, for a US-Letter sized paper (8.5" x 11"), there is about a 1/4" white margin all around the pdf. So the settings are 1/4" 1/4" 8.5" 11", but converted to pix (by multiplying them by 72, or 18 18 612 792).
The first option sets the left white space to ignore in your source image, or top if in landscape mode.
The 2nd option is similar but for the top white space to ignore.
The third option sets the total width to capture from the image, or at least that's what it looks like to me. So perhaps the first option is both the left and right margin. (Sorry, but "I'm not sure. It's very confusing, but I finally got it to work for me.)
The forth option is similar but the height.
You must restart CUPS after you make these edits. Also be careful to keep the file ownership to root:lp. Comments are ok with *% prefix.