This extension is unique in the world of Open Source Software. There is one parameter that is unclear. If one specifies the cover thickness, which in turns changes the spine width, does that number represent the thickness of one side of the spine or both sides? There are choices for PPI, caliper, width, bond weight and so on. But it is not clear whether this assumes that the number entered is for one side of the cover or both sides. For example 360 Digital Books (now owned by Whitlock) provides an number of 0.034 in their spine formula, which represents both sides of the spine.

What does the Inkscape extension expect in that parameter, one side or both?

The true answer will be added to my updated version of the book referenced below, where I recommend the Inkscape cover template for use with Scribus designed covers.

A cover designed exclusively in Inkscape will still need the above question answered.

Why don't I just write a book on an Inkscape designed cover? Patience little grasshopper. That is my next project. Inkscape lacks the ability to produce a PDF document with inherent CMYK color model, and also lacks the ability to produce a PDF X/1-a:2001 formatted document as required by many printers, particularly in the USA. Scribus can do both of these, but lacks a built-in Perfect Bound facility like Inkscape. So my current recommendation is to start in Inkscape and finish in Scribus. Design can be in either one or the other, or both. but that is not the present question, which centers around the thickness of the cover stock.