I am in the process of writing an e-book "Creating Book Covers with Scribus" aimed at the novice user. Early in the e-book I refer the reader to an online facility I created that does the heavy lifting insofar as calculating the overall layout and the necessary/optional vertical guides. The web page should be self explanatory, but you be the judge: http://wexfordpress.net/template.html
The e-book itself is still being written so of course you won't find it online yet. Your comments appreciated. -- John Culleton
