Come to think about it what information do you have in advance about the width of a softcover spine? You cannot rely on any on screen representation of the spine width, it is entirely illustrative.
Spine width depends on the paper thicknesses and to a small degree some elements of the binding process. I have just examined a softcover book produced by another supplier 13 sheets of 280gsm (26 pages) plus cover measures just under 4mm spine width that leaves a maximum of 2mm for any text - i.e. not enough to work with and be legible. In my example the front and back image positions are nowhere near the spine making them less visually critical they are also both 'off' centre' toward the open edge - a cautious approach which would accommodate much wider spines successfully.